Please stop me from selling this...

[rowdyrandy]

Need help diagnosing a stumble at high speed or maybe high rpm. It chugs and bogs at ~4500rpms, stalled on me a few times as well. Seems to die when down shifting and loading the engine but the stumble or miss, I can't tell the difference unfortunately, happens when I really lay on the throttle because if I keep the rpms low and cruise at a slower speed, it won't come around...I've had a trip to the mountains planned for my birthday for a month and its unbelievably frustrating that this has happened a week before hand...

The motor just had a top end rebuild, by me, 1200 miles ago and ran great - you might have even heard me say its the best it's ever run.. - new pistons (1mm over), rings, cam chain, cam bearings, gaskets, valves lashed, valve adjusters, etc..

Things replaced before rebuild since I have owned it - carbs rebuilt (and recently cleaned 2x), clutch replaced, 2 into 1 exhaust, new battery (gel), new points, new brushes, new coils, new turn signal relay, etc..

Consumables have been kept up with i.e. plugs, etc..

Things I've checked -

gas cap venting -
filter and fuel lines -
charging system @ idle & 2500rpms -
brushes -
plugs and wires -
carbs cleaned -
mech advance -
timing -
valves -

Any ideas boys? If not, I'm selling this guy....unfortunately two summers in the garage has shaken my confidence and my urge to ride has overcome my urge to wrench...

If anyone with any insight lives in or around the Denver, Co area, I would love a hand - I'm still fairly new here and don't know too many people. Will provide as many beers as it takes..please help my trip is in a week!!!

 

[DogBunny]

Did you re-jet for your pipes (correct answer is "yes"), and if so, what is the new jetting?

 

[rowdyrandy]

Yup - Like I said, these last 1200 miles have been good..

42.5 pilot - 130 mains - needle at #4 but I've run it at #5 as well - not sure where my mixture screw is off hand

BTW I made the 2 into 1 baffle myself, one of my ideas was that the baffle material was "settling in" or kinda loosing up and maybe that was causing it to run lean...? Seems like a long shot though...

 

[rowdyrandy]

Did you re-jet for your pipes (correct answer is "yes"), and if so, what is the new jetting?

Yup - Like I said, these last 1200 miles have been good..

42.5 pilot - 130 mains - needle at #4 but I've run it at #5 as well - not sure where my mixture screw is off hand

BTW I made the 2 into 1 baffle myself, one of my ideas was that the baffle material was "settling in" or kinda loosing up and maybe that was causing it to run lean...? Seems like a long shot though...

 

[5twins]

It probably took that long to burn up your new topend, lol.

 

[DogBunny]

Your jetting is WAY off. Your bike looks like a 75, right? If you have stock carbs, I would suggest one up on the pilots to 47.5, four up on the mains to 137.5, and I would compensate for upping the main and pilot jets by leaning the needle one clip position to position #2 (second from top). I have no idea how your bike ever ran. I would do a compression check, to see if you possibly damaged the engine due to running lean.

 

[DogBunny]

It probably took that long to burn up your new topend, lol.

Beat me to it. But seriously, do a compression check.

 

[amishmike23]

would love to help but as I'm new here to I'm still learning the ropes so I woukd listen to these other guys. on the other hand I'm just north of you in the Loveland area.

 

[XSLeo]

Also your air filters, what are they? The K&N style pleated element filters, especially the tapered oval or cones leave you with a flatspot just off idle.
The pleated elements cause air turbulance in the carb that effects slide lift. The slides lift by vacumm above the slides. The vacumm comes from the air flow under the slide. The pleated elements disrupts this vacumm signal. This lets the slide flutter instead of open smoothly.
I replaced my K&N tapered oval filters with Uni Filter straight foam filters. No more flat spot. Never ran better.
Leo

 

[rowdyrandy]

remember I'm at altitude too... I'll check when I'm home but maybe I should check the head bolts as well

 

[DogBunny]

I did not account for your altitude, so my jetting suggestions are probably too rich. But, I still think your existing jetting is lean, and that you should do a compression test. What do your spark plugs look like?

 

[TwoManyXS1Bs]

The bike was running fine before, and jetting has to be leaner for the higher altitudes. What we're looking at here is a bonafide incident. Three items come to mind:

1 - ignition, simple check with timing light, not so much at timing but at intermittant/skips in light
2 - vacuum diaphram check
3 - fuel - old/bad/contaminated and wrong season. You've heard of winter blend and summer blend?

Winter blend has a higher reid vapor pressure for cold weather. Summer blend has lower reid vapor pressure so fuel doesn't evaporate too rapidly in the heat. If you have winter blend in summer, you could run too rich. A plug reading would be prudent. Also, during the change of the seasons and new blend deliveries, fuel stations can do funny things. Like push old/wrong blend onto customer, emptying tanks, and getting junk and water in your tank. If your fuel smells like almonds, it's bad.

 

[rowdyrandy]

Alright, thanks for the input guys

Leo - I'm running Uni's, so lets assume no worries there

These plugs don't have a ton of miles on them but it's all I have now - I did a compression test and I've been using a compression table below to compensate for altitude, found it on a toyota forum;

Left 120psi / Alt ~140psi
Right 135psi / Alt ~160psi

I can say there wasn't that much variance last time I torqued the head and checked, is that too large of a gap? Also, timing lights a good idea - I'll do that tomorrow morning.

 

[TwoManyXS1Bs]

Yeah, the compression conversion looks right. Another quickie test for weak ign is to temporarily close the plug gaps to .012-.015".

 

[TwoManyXS1Bs]

One more thing, fuel delivery rate. Pop off a bowl drain plug, something under to catch fuel, with fuel 'on', should get at least 70-100 cc per minute...

 

[grizld1]

A few things. First off, run a bypass to run ignition straight off the battery. If that clears up the problem, check switches and connectors. Check primary resistance on the coils. There's more than 10% difference in compression between the two cylinders, which is not good. Consider doing a leak down test; compression testing will tell you what the highest momentary pressure is, but that's not the whole story.

 

[Scrambled]

Is your 2 into 1 exhaust from Mikes ? Lots of people are having trouble tuning those systems .

 

[Tomterrific]

Hi Randy,

Are you running points? A bad condenser/capacitor can show break-up at higher rpm.

Tom

 

[rowdyrandy]

Well I never got a chance to try half of those suggestions guys but thanks, this beast seems to have reared its head -

last night decided to take it out and try to gauge one more time what was going on. Sure enough it sputtered ~4000 but cruising under that was ok - gave it a fist full of throttle and in 2nd @ 4000 it stalled and wouldn't start again. Walked home, came back and after checking everything out I've got no spark. So assuming this was the problem the whole time, it's electrical...both coils won't fire so it makes me think the problems farther back - batterys 12.6v at rest

 

[TwoManyXS1Bs]

Great! That narrows it down. Quickie/budget test would be to pull both plugs, put them back in the caps and make sure the plug base is grounded, like laying on the head fins. Then, with ign on, finger the points, look for arc at point, spark at plug. No arc/flash at point could mean bad coil or bad power. Bright arc at point usually means bad condenser. Also, point gap may be down. I've seen some newer/cheapo points with prematurely worn-down fibre followers, aggravated by dry cam. Then, of course, there's always the bizarre. But, you know where to look now...