X6 Hustler Bogging at Speed

[Tourmaster]

My 1967 X6 Hustler is running great off the line and all the way up to about 55 mph in 6th gear. If I shift down into 5th gear, I can get it up to about 65 mph ... but under both circumstances she ultimately starts to bog if I try to go faster. Feels like its either running out of fuel a bit ... or perhaps the ignition system is not putting out fully?

I thought that it might be my OEM petcock ... but today, I installed a new aftermarket unit and both clear fuel lines to the carbs are always full up now ... no air pockets. Carbs were both rebuilt with Wemoto components and thoroughly cleaned. Also the float heights were reset to the recommended 25 mm height.

Also, I can blow through the fuel tank cap ... so seems to be venting okay. Also, the carb float bowl vents were both cleaned, so don't think there's any airlocks occurring. Air filter is a bit dated, but appears clean.

Anyways, I'm scratching my head a bit at this point. Anybody have any suggestions?

[GT750Battleship]

Hi,I'm a big fan of those early 250's,you're lucky to have one !Sorry to hear you're having a problem,if ignition is good & not actually misfiring I don't think you have a condenser breaking down,I've heard of points with broken springs & cracked cam feet due to age & lack of lubrication from the felt cam pad..I have friends with T20's, who have said the biggest problem they had with restorations was the carby barrels distort with heat from the motor over time & cause to much clearance for the throttle slide at medium too high rpm. Could be sucking air in somewhere via the air cleaner rubber boot,have a spray round with aero start or something & see if revs pick up at idle,sometimes air filters look o.k. But can still be dirty internally ? Was there any rubbish/dirt in the fuel tank,or old fuel tap How old are the spark plugs & what do they look like,light brown to grey,might give an indication as to a fuel issue ! Have you had the carbys ultra sonically cleaned, & had a close look at the float needle & seat for wear,also if the needle & seat are like the GT750,there is a little filter under it,which can block or disintegrate with age,good luck,I hope you don't find out its the crankshaft seals that have failed,or anything to expensive !!

Cheers,
GT750Battleship.

[Tourmaster]

Thanks battleship. I'm still leaning towards it being some sort of fuel delivery issue. This bike will rev freely, all the way up to red line. Lubed the point cam felt and pivots, and all looks ship shape in the ignition area. Running new plugs and NGK plug caps.

I have a new dual carb boot and OEM clamps, but will check for air leaks by spraying around. I'll see if I can also source a new filter element. Naturally OEM filters are no longer available ... but thought I saw a filter element of similar size at a small engine shop that I might be able to retro fit.

The fuel tank was actually in very good shape internally ... but I have rinsed it out thoroughly, and the new petcock has fine mesh screens on the main & reserve stand pipes.

Your comment about the crank seals has got me wondering a bit also. I've put about 200 miles on the old girl so far, and have been checking the tranny oil level frequently. I assume that any significant smoking and level drop off would be a sign of inner seal problems. Bike smokes on start-up, but settles down once she's warmed up. As I recall having one of these bikes in the late sixties ... that is pretty normal. Anyways, I'll keep checking all the simple/easy stuff first. Worst case scenario, I'll have the seals replaced with a new set I picked up from a fine gentleman in Australia.

Thanks for your suggestions.

Cheers
Bob

[yeadon_m]

Bob,
I wonder if you have the correct main jet size? it may be worth trying a size or two larger to see if better or worse.
My GT550A symptoms sounded a bit like your description. I'd followed what I thought was the correct jetting from (some) manuals, only to find by experiment that my carbs required bigger mains, after which power on road at wide open throttle has leaped upwards.
Cheers,
Mike

[Suzukidave]

Would opening the choke a bit when its bogging be a test for lean top end ?

[Tourmaster]

If changing out the air filter doesn't do the trick, I like the idea of trying a bit larger main jet ... wouldn't hurt to try. By the way, to repond to your float needle comments, my needle and seats are new Wemoto items ... no screen in them, and they look just like the OEM units. The other suggestion of running with the choke on ... I'll give that a try. By the way, the choke on the T20 Mikunis are prime plunger style, not choke plates ... anyways, nothing to lose and easy to try.

Again, many thanks for the helpful suggestions.

While we're on the topic of carbs, when I disassembled my stock Mikuni VM24-SH units, and cleaned the various parts, I ended up with a couple of questions concerning the airbox side carb body throat ports. The detail below was asked on the t20suzuki.com forum over a month ago, but no one answered. Got over 100 hits, so lots of interest ... Here's what I posted below:

In this photo, I've placed the red pointer on the bottom center port. This one is connected to the air screw port and is perfectly clear. However, in one of my generic Mikuni carb books, it shows that this port can also connect straight through to the main needle jet bore. But both of my carbs are seemingly blocked from this path. Does this port have no passage way through to the needle jet area on these particular carbs? Or, do I need to keep working on this to clear it?



In this photo, I set the red pointer on the lower left port. This port on both my carbs are open for almost 5mm, but do not seem to go anywhere. They certainly aren't open to any other area of my carbs at this point. Is this a deadended port, or is it supposed to open to another area of the internal carb body?



This photo is just for reference showing my red pointer on the lower right side port which is blocked with a brass cap.



I sure would be nice to get an answer to these port questions, as it may, or may not have a bearing on this top end bogging issue.

[yeadon_m]

I have no experience on these carbs but have many Mikunis on Suzuki GTs. All have at least two air inlets, one for the pilot and one for the main system. I can't understand how it could run those two circuits off one port, unless somehow they are siamesed inside.
Can you squirt carb cleaner in the hole(s) and watch spray appearing at motor end or at least as a spray inside the engine side of the carb?
As a note, my GT550A carbs have one blank hole like you describe - machined but goes nowhere (the bike runs fine on pilot and main).
Interesting....
Cheers,
Mike

[Tourmaster]

Thanks for the feedback Mike. I've poked, prodded and sprayed carb cleaneer through every possible orifice on these carbs. But those passages just won't come clear. Hesitant to get real aggressive with the poking (I have a carb wire tool) for fear of damaging something. I was a bit disappointed that no one on the T20 forum had an answer to this.

The pilot and main passages on my carbs are open via other routes ... just not through these airbox side ports. But I've rebuilt and cleaned enough bike carbs in my time to know that many jets are linked to multiple passages. Sure would like to hear from someone familiar with these T20 VM24's specifically.

[Suzukidave]

I wonder how close they are in design to the 380's vm24 carbs http://www.oldjapanesebikes.com/mraxl_G ... cover.html" onclick="window.open(this.href);return false;

[Tourmaster]

The top end of those carbs are certainly different than mine. And they don't go into any detail about passages and what to look for when cleaning them. Upon reading through that link. however, it was interesting to note that the float height could be an issue at higher throttle positions. When I cleaned and rebuilt my carbs the float heights were closer to the 27-28 mm mark before I tweaked the tangs, resetting both to 25 mm ... which is the recommended height stated in my T20 shop manual. I see the GT380 manual for the VM24-SC version states a 25.7 mm height. hmmmmm ... thanks again for the ideas Mike.

By the way, just came back from another test ride ... this time removing the fuel tank cap to eliminate the venting possibility. Still bogged at about 55 - 60 mph. Otherwise, the bike runs beautiful. For example, bike sat for 2 weeks ... and took only one kick. Same when the engine is warm ... almost always one kick. Idles nice, strong acceleration off the line .... albeit, slightly less responsive after she fully warms up. I seem to recall that our Hustler on the farm 40 yrs ago, was the same way.

I'll keep digging ... but if there's anybody out their who is familiar with these specific carb ... feel free to chime in.

[joethebike]

On my 1972 T350 the high speed bogging issue was fixed by going DOWN in main jet size from the standard 112 to 107. Plug colour is good, no pinging and the best thing, it fixed the problem.

[Suzukidave]

Not saying you set them wrong , but .. the diagrams showing setting float height show the carb straight up and down . I truth you are suppose to tip the carbs till the tang on the floats just contacts the tip on the float valve and check the height there .

[Tourmaster]

Well, that's interesting on the jet sizing. My stock jets are 95's ... the Wemoto kit came with some 90's also ... perhaps I'll give them a go. In support of that, I just pulled my new plugs and they are dark brown and a bit sooty. Called my local bike shops and there are enough up and down jet sizes on hand to run through some jet tests. Will start with the 90's and see where this leads.

With regards to the float height, I'll double check them when I go to rejet. You're right, I think I had them upright, so may have been some deflection on the spring loaded float needle. I'll make sure I get them spot on this time. The devil is always in the detail.

Cheers guys .. Many thanks

P.S. I'll keep y'all posted in a day or two, as I work through the float height and jetting issues.

[Tourmaster]

Okay ... so I have some good news, and some bad news.

Finished diassembling, recleaning and installing the #90 main jets. Also, rechecked the float heights with the carb body at the correct angle ... actually tweaked the right side a bit. Reinstalled everything and took her out for a rip. Pulled out on the highway, cranked it up to 70 mph cruising speed, and with the exception of a couple of very brief/minor hesitations ... maintained that speed for 3 miles before turning off on our paved backroad I use for testing. Seemed like I could have easily gone faster, but didn't want to press my luck with the somewhat dated tires. This was a definite improvement in performance.

Once off on the backroad things continued to go just fine for about another 5 miles, and then the darn bogging resumed. Pulled over and noticed the idle revs were now up around 3000. Took a look and sure enough the left throttle slide collar had worked itself loose. Tighten it back up, revs returned to 1500 ... and so I resumed my cruise. Unfortunately, seemingly now with the engine fully warmed up/ fairly hot ... I was back to the same old bogging at about 50 - 55 mph.

I'll let the bike completely cool down, and expect I'll probably be able to reach the higher speeds again until the engine gets fully warmed up again.

Pulled the plugs again ... left one is a perfect beige/brown, with no sootyness. But the right plug looked a bit different .... a bit of brown on the electrode, but the outer circumference is almost clean shinny metal. Certainly a bit cleaner looking than the left plug overall?

By the way, I'm running NGK B7HS plugs, not the OEM B77HC's. My local dealer indicated that the B7HS plugs are a comparable replacement. I'm mentioning this, just in case any of you have a different experience/opinion.

I'm obviously encouraged by the improved performance overall, but still somewhat frustrated with the continued bogging upon full warm up ... and also the mismatch of plug appearance. Hmmmmm

[Tourmaster]

So, I've been through just about everything I can think of. I even double checked my points and timing today. Actually, ever so slightly, advanced the right side. Both left and right gap was spot on owners manual recommended 0.35 mm. More firmly tightened the throttle body "chamber" caps. Double checked the throttle slides to ensure evenly balanced.

This bike actually runs great for about the first 20 minutes. One kick start-ups, even idle, strong acceleration and cruises smoothly at 70 mph ... with room to spare. If the bike ran like this on a sustained basis, all would be great. But, once she's fully warmed up ... getting a bit on the hot side ... its back to the bogging at full throttle and a limited top end speed of about 55 to 60 mph.

A couple of things come to mind. First, the easy one ... as I mentioned I'm running B7HS plugs. Would the stock B77HC plugs be somewhat colder? Should I try running colder plugs? If so, what's a suitable currently available NGK version?

Although the engine appears to have good compression, could this hot runnig be related to piston/cylinder clearances beyond service limit? Is this condition telling me it may be time to consider oversized pistons?

Or ... is there something else that might explain this loss of performance after full warm-up?