T500 carb puzzled!

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ro55o
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Re: T500 carb puzzled!

Post by ro55o »

Just checked battery again = 13.12........... not used since Sunday.
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dorT500
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Re: T500 carb puzzled!

Post by dorT500 »

ro55o wrote:Just checked battery again = 13.12........... not used since Sunday.
An 'at rest' voltage reading like that on a acid-lead battery would give me pause to wonder a bit but this must be a new AGM battery you are using?

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ro55o
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Re: T500 carb puzzled!

Post by ro55o »

Yes, thats right.
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Re: T500 carb puzzled!

Post by Vintageman »

Just read again some of this post.


Shims under needle clip can be a problem sometimes. The Needle needs to float freely in the Slide so it self centers itself in the needle jet. I have seen at times these shims cause the needle to be stiff as it makes clip too high and gets tight for it presses up to needle retainer. Just another headache to avoid.


Wow 150 round main on pre 1973 T500. I have a 1975 came 97.5 main. The needle is the 5FP17 not the 5fp8. Never compared needles, but suspect the 5FP8 is thicker at WOT position. I know the early carb is Homo venting scheme ... suppose help low end richness (per Suz I think they show this). The air box is different I know too between early and late. Same on GT250 and I have run both.

Well if stock is 150 round than that is what it is to be. ConnerVT has this year and he verified.

Check the bowl venting air passage to see if is well opened and no sand bugs plugged that passage or thinned it along the way. same with needle jet air passage. not enough air will make it rich. I would tune on stock air box with clean filter until you sort this out but just me....

Let us know what you find.
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ConnerVT
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Re: T500 carb puzzled!

Post by ConnerVT »

Vintageman wrote: Wow 150 round main on pre 1973 T500. I have a 1975 came 97.5 main. The needle is the 5FP17 not the 5fp8. Never compared needles, but suspect the 5FP8 is thicker at WOT position. I know the early carb is Homo venting scheme ... suppose help low end richness (per Suz I think they show this). The air box is different I know too between early and late. Same on GT250 and I have run both.
The late model 5FP17 is a little richer than the 5FP8. I measured the two awhile back, and if I remember correctly (not a given these days) the main difference is the 5PF17 starts its taper a bit sooner. The top of the 5FP8 may be slightly thicker, but this I'm less sure of. Near the tip, they are about the same.

The big difference in the main jets is really understandable. The later carbs have the float bowl (the more positively pressured input side of the jet) at atmosphere, as it is vented to the outside world. The early homopressure carbs are vented to the intake port side (toward the cylinder head) of the carb. This is at a lower pressure than atmosphere.

The size of the jet needs to be larger in the homopressure carb, as the pressure in the float bowl is less than the carbs vented to atmosphere.
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Re: T500 carb puzzled!

Post by Vintageman »

Yes ConnerVt makes sense but why is it so ->

https://www.tapatalk.com/groups/kawasak ... t1890.html" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Search on "Homo" in the link above


This is good explanation me thinks. What I think it says, regardless of factory air box design/size pressure drops inside the air box always occurs when air flow increased so less than atmosphere at bell mouth.

What confused me was the fact the vent hole is right right in the Bell mouth of carb near exit and I could not see how that could really be very different pressure than atmosphere just an inch away. What I did not include in my mind's eye was the airbox attached and drop in pressure it can causes. I was thinking about it with nothing attached to carb bell mouth.

So if you are running real efficient pods with no real pressure drop, well then, the pressure is less than Suz expected with their airbox ...more towards full atmosphere. Wow if that is true, explain why you would be so rich. You sure your tried stock airbox with clean filter?

I suppose if anyone has used early style carbs with PODs and stock jetting or richer proves me wrong
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Re: T500 carb puzzled!

Post by ro55o »

The air filter is brand new, so defo clean.
Currently it runs really badly with the air filter and much better with pods, Ive tried this a good few times also with totally stock jetting.

Also will hardly even start with the new 159 needle jets...with pods or with air box.

I have also got on the bike........... NEW:
points, condensors, coils, ht leads, plug caps, plugs, battery, new wiring to stator, new air filter boot.
All oil seals are new in the engine, crank rebuilt, pressure tested and is good.

I have been through as many areas as I can think off over the last year and nothing has made any difference, it still bogs at half throttle.

I know my timing has slipped just recently............ but I have ensured in the past that I have had the correct points gap, plugs gap and timing set with dial gauge.

I am just re cleaning carbs at the moment to try again asap. I have checked all air passages and as far as I can see are clear.
Of course I cant be 100% sure of that but hit the with the compressor and its blowing through.
I didnt get a response to the question about the air holes at the bell mouth (earlier post), but Im pretty certain the ones I encircled in red are supposed to be blocked off.

The carbs have original jetting in them except 152.5 main, and needle in 2.
When I tried the needle shimmed it was purely to investigate if there was any difference/improvement and I then went back to clip 2.

The bike came with a bag of original carb parts and a keyster rebuild kit.
Having read the keyster kit is no good I didnt use it ...except for the chokes.
So Im going to try the original chokes although I dont expect that to help.

Concerning the float needle and seat ..are the keyster ones OK?

I havent used them but they look OK so might see if that helps as they are new.
I have some original rubber tipped needles (which I havent tried), but the little plunger doesnt move up and down, I presume they should? or are they different to normal.

Other thoughts are.....has there been some porting work? that would affect the running, so Ill try and find the info on that and maybe investigate at a later date.

Also has the left side timing plate been slipping all the time? The lower bolt holding the points plate was stripped so I re-tapped it but its just gone again so I need to address that.

Vintageman you mention that the T500 can slip its keyway...how exactly, and how can you check to see if it has.
My timing marks are almost the same as the dial gauge readings so I presume its OK.

Thanks for the interest and suggestions so far, Im going to keep going through things and checking, Ill get there in the end one way or another!
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Re: T500 carb puzzled!

Post by Vintageman »

ro55o wrote:Concerning the float needle and seat ..are the keyster ones OK?
I have used. The only issue is sometime you really have to bend the float tab more than typical to set float height. I'll add extra OEM washer to to shim it up higher if what is needed. At one point I found just the mikuni needle and now just replace that (can't find part # now) polish up brash body if no damage. expensive item OEM


I am late to post. Did someone already ask to be sure you did not swap left and right pistons.

Look into intake and exhaust and piston and walls look nice... nothing bad is happening with fitment?

Porting... the specs are like the later GT750... forget that too now but its posted somewhere here (one of my posts?) IF you look into intake or exhaust you can typically tell if someone ground Aluminum and Casting first clue is they match evenly
ro55o wrote:My timing marks are almost the same as the dial gauge readings so I presume its OK.


If true for both sides then all is OK. My T500 bike had spun keyway and one of the spare engines I have did so maybe common. I think it happens if the engine, during starting (lame kick attempt) back fires and reverses quickly. The crank has a lot of mass. This is one of those problems some people never figure out (why my bike was setting for so long and very nice cosmetics). One old wise mechanic told me there is always a reason they get parked like that. The other issue is loose or bad condenser. Someone will pay to have it tuned for it runs bad pops and sputters. Works for short will and comes back. Those service sometimes don't check that and just do points. The bike is nice, so just gets parked

This is your first time trying to get it going? Well you have done much work so odd anything left.

Do you think it is rich or lean when it bogs? does it ever backfire?

Well do those things you said, maybe take video. I wish you luck. I am confident you will find and please do tell.

I have stripped points screw too, easy and screw not long enough first few threads strip tap to larger dia threads

I just had an odd issue, running my gt550, long ride, accelerated hard 10 miles from home hard then all of the sudden ran like poo and would not take more than 1/4 throttle. Felt very rich (gurgle crackle before it cut out, once it cut out even backing off took a while to finish the extra fule). To be honest It is hard for me to tell which it is lean or rich. Checked plugs, ignition, gas cap venting, fuel flow, compression then started to take off air box and found my registration got suck into the snorkel of the airbox. Very rich indeed

Good luck
Current Bikes
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76 GT250 (T350 upgrade),
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74 GT380,
75 T500,
73 GT550,
75 GT750,
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77 Yamaha RD400 (Daytona Cyls),
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Re: T500 carb puzzled!

Post by Vintageman »

ro55o wrote:Concerning the float needle and seat ..are the keyster ones OK?
I have used. The only issue is sometime you really have to bend the float tab more than typical to set float height. I'll add extra OEM washer to to shim it up higher if what is needed. At one point I found just the mikuni needle and now just replace that (can't find part # now) polish up brash body if no damage. expensive item OEM


I am late to post. Did someone already ask to be sure you did not swap left and right pistons.

Look into intake and exhaust and piston and walls look nice... nothing bad is happening with fitment?

Porting... the specs are like the later GT750... forget that too now but its posted somewhere here (one of my posts?) IF you look into intake or exhaust you can typically tell if someone ground Aluminum and Casting first clue is they match evenly
ro55o wrote:My timing marks are almost the same as the dial gauge readings so I presume its OK.


If true for both sides then all is OK. My T500 bike had spun keyway and one of the spare engines I have did so maybe common. I think it happens if the engine, during starting (lame kick attempt) back fires and reverses quickly. The crank has a lot of mass. This is one of those problems some people never figure out (why my bike was setting for so long and very nice cosmetics). One old wise mechanic told me there is always a reason they get parked like that. The other issue is loose or bad condenser. Someone will pay to have it tuned for it runs bad pops and sputters. Works for short will and comes back. Those service sometimes don't check that and just do points. The bike is nice, so just gets parked

This is your first time trying to get it going? Well you have done much work so odd anything left.

Do you think it is rich or lean when it bogs? does it ever backfire?

Well do those things you said, maybe take video. I wish you luck. I am confident you will find and please do tell.

I have stripped points screw too, easy and screw not long enough first few threads strip tap to larger dia threads

I just had an odd issue, running my gt550, long ride, accelerated hard 10 miles from home hard then all of the sudden ran like poo and would not take more than 1/4 throttle. Felt very rich (gurgle crackle before it cut out, once it cut out even backing off took a while to finish the extra fule). To be honest It is hard for me to tell which it is lean or rich. Checked plugs, ignition, gas cap venting, fuel flow, compression then started to take off air box and found my registration got suck into the snorkel of the airbox. Very rich indeed

Good luck
Current Bikes
74 GT250 (T350 upgrade),
76 GT250 (T350 upgrade),
71 T350,
70 T350,
74 GT380,
75 T500,
73 GT550,
75 GT750,
72 Yamaha DS7 (R5 upgrade),
77 Yamaha RD400 (Daytona Cyls),
Vintageman
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Joined: Tue May 18, 2010 5:38 pm
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Re: T500 carb puzzled!

Post by Vintageman »

ro55o wrote:Concerning the float needle and seat ..are the keyster ones OK?
I have used. The only issue is sometime you really have to bend the float tab more than typical to set float height. I'll add extra OEM washer to to shim it up higher if what is needed. At one point I found just the mikuni needle and now just replace that (can't find part # now) polish up brash body if no damage. expensive item OEM


I am late to post. Did someone already ask to be sure you did not swap left and right pistons.

Look into intake and exhaust and piston and walls look nice... nothing bad is happening with fitment?

Porting... the specs are like the later GT750... forget that too now but its posted somewhere here (one of my posts?) IF you look into intake or exhaust you can typically tell if someone ground Aluminum and Casting first clue is they match evenly
ro55o wrote:My timing marks are almost the same as the dial gauge readings so I presume its OK.


If true for both sides then all is OK. My T500 bike had spun keyway and one of the spare engines I have did so maybe common. I think it happens if the engine, during starting (lame kick attempt) back fires and reverses quickly. The crank has a lot of mass. This is one of those problems some people never figure out (why my bike was setting for so long and very nice cosmetics). One old wise mechanic told me there is always a reason they get parked like that. The other issue is loose or bad condenser. Someone will pay to have it tuned for it runs bad pops and sputters. Works for short will and comes back. Those service sometimes don't check that and just do points. The bike is nice, so just gets parked

This is your first time trying to get it going? Well you have done much work so odd anything left.

Do you think it is rich or lean when it bogs? does it ever backfire?

Well do those things you said, maybe take video. I wish you luck. I am confident you will find and please do tell.

I have stripped points screw too, easy and screw not long enough first few threads strip tap to larger dia threads

I just had an odd issue, running my gt550, long ride, accelerated hard 10 miles from home hard then all of the sudden ran like poo and would not take more than 1/4 throttle. Felt very rich (gurgle crackle before it cut out, once it cut out even backing off took a while to finish the extra fule). To be honest It is hard for me to tell which it is lean or rich. Checked plugs, ignition, gas cap venting, fuel flow, compression then started to take off air box and found my registration got suck into the snorkel of the airbox. Very rich indeed

Good luck
Current Bikes
74 GT250 (T350 upgrade),
76 GT250 (T350 upgrade),
71 T350,
70 T350,
74 GT380,
75 T500,
73 GT550,
75 GT750,
72 Yamaha DS7 (R5 upgrade),
77 Yamaha RD400 (Daytona Cyls),
ro55o
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Re: T500 carb puzzled!

Post by ro55o »

Pistons are correct way round.
Bike came to me in boxes, so wasnt running when I got it, and so have no reference points other than the manual settings.

Ive been going through the whole bike fixing problems as they arise, so although its been running for a year I still havent got it running right. So yes its the first time its run (for me), God knows where its been and whats been done to it.

Yes it is tricky to be sure on rich and lean so Ive spent alot of time trying to be sure, (although I could still be wrong).
Bike never backfires.
Ive needed to lean out needle position which suggests rich. Wont pull after half throttle very well, if theres a hill its pronounced.
With the air box/filter it would almost stop dead if there was a hill, no power after half throttle.
I started rich on the main jet 160 and worked way down, much too rich to start with.
Pulling choke makes it worse.
Plugs can oil up and are blacker than ideal.

All suggesting not enough air to me, thats why the suggestions to check air passages in carb etc.. seem reasonable and I will be checking these possible problem areas again so Im double sure.

Bike will run well to half throttle though, hence the thoughts about problem being needle jet/needle.
Also the original question regarding why in my case the 159s are so much worse.

Ill get it sorted eventually and Ill re-post know when I figure it out.
Cheers for now.
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Re: T500 carb puzzled!

Post by Vintageman »

your original questions about why 159s are run different. First I would say check your 188s. Check them with a bright light (sun light works well too) sent up the tube and you look in with a magnifying glass whee needle inserts. If it the very first few mm, where it meters, is must be precise, smooth if not shiny. Due to H2O in our atmosphere and condensation on the needle, I have found it to look like the surface of the moon , pitted at best if orginal. I would think first stage of pitting would cause extra restriction to fuel and be leaner where as total loss of material would be richer as diameter larger. So your 188 may have been making it run leaner. Check,,, if nice put them back in, but if not stay with those 159s is my advice. Oh, make sure you use Mikuni main Jets the other are know to have at time quality problems

What will happen when the surface is beginning to pit, mid range does not run smooth and engine won't purr like it should in fixed positions. When you have the two of exact same bikes or snowmobiles one is pristine the other is not you will find the one that is perfect is so much enjoyable to ride where the other you may not realize you always almost making minor adjustments to throttle position, not fixed, not enjoyable for that reason. I religiously change the needle jet if not perfect, most time they are not, but my area is does have moisture humidity and condensation.

My present T500 has the 159s and with Hex conversion not rounds as I thought at first... I found the conversion chart not accurate but eventually tuned the bike well. I Run stock air box and Jemco chambers. When I just went one size size larger it would not run above half throttle... gurgled. Like a light switch! I knew I went too far and was too rich. If you don't find anything wrong it won't hurt to try a leaner jet to see if it cleans up if a size or two less than stock.

I have a few T350 today as well running and think all have 159s and round jet. They are stock jetting I run stock airbox and chambers. The USA mpg is low 40s.


You do sound rich, why?, maybe you are just a one or two size to0orich and light switch as I described.
Current Bikes
74 GT250 (T350 upgrade),
76 GT250 (T350 upgrade),
71 T350,
70 T350,
74 GT380,
75 T500,
73 GT550,
75 GT750,
72 Yamaha DS7 (R5 upgrade),
77 Yamaha RD400 (Daytona Cyls),
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