Suzuki 2 Strokes

tz375 wrote:Massive reversion seems to be picking up fuel each time it passes the jets. I don't recall the inlet timing on that motor - it could be too long.

You could also try even smaller pilot jets but that may make the transition to the needle & jet a problem, but one step at a time. How big is the air "jet" that feeds the pilot jet? It should be a fairly large drilling. And air blockage in there would exacerbate the situation

Dang Richard I wish I lived near you so we could work through it together! I don't understand the air jet drilling, is that where the air screw goes into?

John,
I checked through the pics of the build and found your carbs and they look OK in that department. You said it starts OK without choke and idles OK so that actually suggests the pilot jet is a size too large. Main is OK so that suggests one of two things.

Sounds like the needle and needle jet combo is way too rich OR the choke plungers are not seating properly. I found some on a GT750 where the rubber tip of the choke plunger was hard and did not seal properly. It would be worthwhile to check that out. If that's all OK, try a smaller needle jet or thicker needle to see if that makes a difference.

Hi john,

My 2c worth is that the fuel blow back you are experiencing is a normal factor for a piston ported engine. It will be more pronounced if you have advanced the inlet porting with a bit of tuning but will probably almost disappear when the revs rise and the engine resonances begin to work. My race engines do this and you learn to live with it, but it's never too bad. You could always convert to Reed Induction and if you make a few kits I'll be your first customer

Cheers Geoff

Hey guys finally got back to working on the 500 and I seem to have it running halfway decent, it is way better than it was before I broke the golden rule and switched two things at once but I think it was a combination of the two anyway. I bought brand new coils and a dual outlet Pingel gas tap. As i say it is running much better now but still rich on the bottom.
Now I have actually been able to ride the bike more than a 1/4 of mile I have realized that the suspension is way too stiff, the handlebars are too far forward, seat is too low and as Teazer pointed out the gearing is too tall. All easy stuff to rectify I think!
Here are a couple of very short video's from last week, I will get better video soon
https://youtu.be/NNO78cQYQxo" onclick="window.open(this.href);return false;
https://youtu.be/h5LEiQw5wNE" onclick="window.open(this.href);return false;

Well it's been a long time since I posted anything but have finally started to work on the 500 again. I will admit that I basically got so frustrated that I just left it sitting and built another bike (BSA b50mx). However I have got the itch to sort it out once and for all. So a week ago I removed the carbs and totally cleaned them. Drained the old gas and decided I would use a different oil, Maxima super m (was Castrol 747 before) at a different ratio, gone from 25:1 to 32:1. When I started it last week I thought I had cured everything as it ran and sounded great. I didn't have time for a test ride however. So yesterday, one week later I fired it up to go for a test ride and immediately I could tell it was not running as well which was borne out on my test ride. I made video's of both times I ran it. I am going to look for someone in my area that does dyno work on two strokes as I am completely confused. It runs ok when you get high up in the rev range.
Current setting are:
Main jet 270
Pilot 30
Slide cutaway 2.5
Needle is second groove from the top
Plugs br9hs
Sunnoco 110 leaded race gas mixed at 32:1 with Maxima Super M
https://youtu.be/MBKEDNHKPLM
https://youtu.be/PzYMtSS8mUM

First run is smokey but sounds crisp and the second run sounds flat. What changed other than the oil? Is it the same type of gas on both runs? What else changed that you forgot about, such as weather.

Check the Dynojet website for locations that might be close enough to you or call local dealers and see if anyone can suggest a dyno shop.

tz375 wrote: ↑Sun Oct 31, 2021 4:27 pm First run is smokey but sounds crisp and the second run sounds flat. What changed other than the oil? Is it the same type of gas on both runs? What else changed that you forgot about, such as weather.

Check the Dynojet website for locations that might be close enough to you or call local dealers and see if anyone can suggest a dyno shop.

Actually I may having been a little confusing about the oil ratio, both runs were same oil and gas at the same ratio 32:1. Weather was very similar both times.
I will check into Dynojet

Wonder if it's using petrol/oil mix in the carbs as they were originally designed for petrol only and an oil pump for the 2T?
Just a thought......

johnu wrote: ↑Mon Nov 01, 2021 1:39 pm

tz375 wrote: ↑Sun Oct 31, 2021 4:27 pm First run is smokey but sounds crisp and the second run sounds flat. What changed other than the oil? Is it the same type of gas on both runs? What else changed that you forgot about, such as weather.

Check the Dynojet website for locations that might be close enough to you or call local dealers and see if anyone can suggest a dyno shop.

Actually I may having been a little confusing about the oil ratio, both runs were same oil and gas at the same ratio 32:1. Weather was very similar both times.
I will check into Dynojet

OK, so what happened between runs if nothing changed unless a jet is clogged or you have water in the fuel which is quite possible. Or did the oil separate out and partially foul a plug. Try fresh plugs and see if it clears its throat. If that doesn't work, drain the float bowls and tank and try fresh gas.

I forget what ignition you have on that bike. Could battery voltage be low by any chance?

tz375 wrote: ↑Mon Nov 01, 2021 9:11 pm

johnu wrote: ↑Mon Nov 01, 2021 1:39 pm

tz375 wrote: ↑Sun Oct 31, 2021 4:27 pm First run is smokey but sounds crisp and the second run sounds flat. What changed other than the oil? Is it the same type of gas on both runs? What else changed that you forgot about, such as weather.

Check the Dynojet website for locations that might be close enough to you or call local dealers and see if anyone can suggest a dyno shop.

Actually I may having been a little confusing about the oil ratio, both runs were same oil and gas at the same ratio 32:1. Weather was very similar both times.
I will check into Dynojet

OK, so what happened between runs if nothing changed unless a jet is clogged or you have water in the fuel which is quite possible. Or did the oil separate out and partially foul a plug. Try fresh plugs and see if it clears its throat. If that doesn't work, drain the float bowls and tank and try fresh gas.

I forget what ignition you have on that bike. Could battery voltage be low by any chance?

I think it is possible that it could be one of the plugs. The fuel is fresh just mixed one week ago and I sloshed the tank around before starting the bike to make sure that the oil and gas were mixed. It has the GT500 electronic ignition. I will clean carbs again and put new plugs in and see what happens. I have found a two stroke expert in Phoenix AZ with a dynojet, it's a five hour drive each way but probably worth it.

Alan H wrote: ↑Mon Nov 01, 2021 6:01 pm Wonder if it's using petrol/oil mix in the carbs as they were originally designed for petrol only and an oil pump for the 2T?
Just a thought......

Thanks Alan but I am not using the stock carbs these are VM34 mikunis.

An original GT500 CDI might be an issue. CDI 'black boxes" do fail over time. I think it's a deterioration of the capacitors, but that's not my area of expertise. Rex's speed Shop in the UK has replacements IIRC and someone did post the schematic on this forum many years ago.

johnu wrote: ↑Tue Nov 02, 2021 11:35 am

Alan H wrote: ↑Mon Nov 01, 2021 6:01 pm Wonder if it's using petrol/oil mix in the carbs as they were originally designed for petrol only and an oil pump for the 2T?
Just a thought......

Thanks Alan but I am not using the stock carbs these are VM34 mikunis.

I meant that the petroil mixture will give a different burn characteristic to just petrol in the carbs.
Curious to why you didn't use the oil pump though? Is it just for a cleaner look under the seat area?
Are the oil slingers still on the crank, or doesn't the 500 have them like the triples?
(My experience is mainly with 550s which don't last long on premix unless there's a lot of crankshaft work done - removing the slingers which shift oil away from the main bearings towards the little ends and under the piston crowns - and don't allow oil into the bearings which are pressure fed.)

Hello johnu,

Just a follow-up on plugs and their replacement. When testing a new race engine on the Dyno, which was expected to be in the mid-60's RWHP the first run produced a measley 44 RWHP. A bunch of jetting, timing changes etc. tried with no improvement. Just before we packed up and were going home in puzzlement I said "lets sling a new set of plugs in". Result, an instant 60+ RWHP. These runs were done within minutes of each other with NO other changes. The plugs in use were NGH B10EGV''s an Iridium plug. The Dyno guy (very experienced in 2 and 4st testing) was of the opinion that the Iridium plugs were good in 4-st's but they were not the best for 2-st's. Even worse once they had gone off no amount of cleaning would bring them back to peak performance. A new set of plugs is a pretty easy thing to test. Worth a try.

Cheers Geoff

Thanks Geoff, going to pick up new plugs today! I have been running BR9hs plugs.