WOT breaking up a bit 94A Wombat

[944turbo]

Hi guys my 94A wombat starts great, idles great, runs thru all gears up to fifth great but at WOT it bogs on slight upgrades and hills. I changes main jet from 140 to 150. Do I need to go larger or smaller on the main jet. Thanks

[dirty_rat]

That's hard to say. It could be any one, or any combination of things. It could be from not being jetted perfectly, and it could be from not having the proper gearing for what you want it to do or it could be a clogged pipe or restrictive intake. At what speed (MPH) is it bogging? It could be geared to tall to pull the gearing, your weight and the incline that you are attempting. More information is needed to make any better assessments.

[944turbo]

This happens 35/40 mph on the road. Gearing is 14/56. Pretty sure intake is not the issue or for that matter a clogged pipe, I Judi installed a new air filter and checked the pipe

[matt glascock]

A systematic approach is the best and most efficient approach to sorting out this issue. As D.R. pointed out there are many potential problems that lead to the symptoms you mentioned. Try beginning with a thorough cleaning - inside and out of the motorcycle. That means wash the bike and clean and re-oil or replace air filter. Remove and perform a complete carb cleaning and reassemble to STOCK jetting specs if the bike has stock induction and exhaust. Burn out the pipe and spark arrestor and pull, decarbonize and reinstall exhaust outlet. Fill the tank with FRESHLY pumped gas properly mixed to 32:1 with race-grade two-stroke premix oil (Yamalube 2R seems to be the sentimental favorite) and replace plug with stock recommended plug properly gapped. Check, clean, and reset points gap to spec. In other words, spend the time to systematically return the bike to stock specs and eliminate all the variables that can adversely effect operation. Then when you begin jetting runs and other diagnostics the information revealed will be meaningful and actionable. There are excellent treatises on the role of jetting runs on the tuning of Mikuni carbs available all over the net . First though, quite frequently simply getting the bike back to spec alone will solve the problem. Get yourself an official Wombat Shop Manual (available on this site). Great systematic descriptions of how to tune the Mikuni carb and all other aspects of maintenance and repair of your bike. I have one and with it in hand, have adequately convinced myself I'm a real dirt bike wrench. Good luck!

[matt glascock]

oops...I see you've addressed some of these things while I was rambling. Still, what I said is valid. Eliminate variables and be systematic and methodical in your diagnostics and you will be much more efficient is identification and correction of the problem.

[944turbo]

Yeas I did everything you mentioned including the timing,new plug and rebuild of the carb. When went from the stock 140 jet to 150 I felt like the bike ran better across the lower rpm range. I think I'll try a 130 to see if that makes the bike run better. I'm a bit concerned about running too lean.

[socalhodaka]

Make sure the o-ring that seals the main is, seals into your float bowl. It will have you chasing some jetting issues.

[944turbo]

I did a carb rebuild so the o ring is brand new. Thx for the suggestions. I think I'll persue the 130 main jet and see were it takes me

[Bullfrog]

I'm wayeeeee spooked about going to the 130 main jet. Going leaner is the direction which results in seizures and I don't recommend it in this situation without some more clear-eyed diagnostic information. I am especially concerned about this comment - ". . . from the stock 140 jet to 150 I felt like the bike ran better across the lower rpm range." My concern is on several levels:

You report the bike ran better with a richer main jet . . . so you are going to try a leaner one? (not recommended)

You report that changing the main jet improved things at lower rpm - engine rpm is mostly not relevant when jetting. What counts is throttle position. The main jet is the primary determinant of air/fuel ratio at 3/4 to full throttle. Changing the main jet should have virtually no effect on engine operation below about half throttle. Since most lower rpm engine operation happens at a lower throttle settings, I fear we may have a situation of "hopeful error" - as in "I changed something to make things better . . . and I think, maybe, possibly they actually did get better". Don't let "hopeful error" lead you astray.

You didn't report whether things got better at full throttle when you changed from the 140 to the 150. We really need to KNOW that little bit of crucial information. If you have an engine operation problem at WOT, the main jet is indeed the proper thing to tune . . . be we need to know what effect the main jet change had. Did things get better or worse?

I hope I'm not too late to stop the 130 main jet test for the time being.
Ed

[944turbo]

Ed I agree it didn't make sense to me to change to a 130 MJ. I do think there was less bogging down with the larger jet(150) than the standard 140. In fact I sure there was less bogging down. I do have a question on timing, I replaced the points and condesor but is it possible to not be correct on the timing? The bike runs well in all of the throttle positions other than 3/4 to WOT, if I back off the throttle I can get the bike to run better and gain speed without break up. It takes longer to get up to speed on a road and I don't like cars running up my rear end. Otherwise I'm happy with all other throttle positions.
I think I have most of the normal settings taken care of and I sort of think I'm dealing with a jet issue. I don't want to add this just yet but would there be similar symptoms if the crank seals have gone south? The engine has about 1000 miles on it and has certainly never been opened up

Thx, Tom