Why won't my bike hold an idle? (700cc motor)

[blackbetty]

I've been trying to tune my 700cc motor and I'm getting close, but I have a recurring, nagging issue.

Sometimes during idle, my bike will "burp" and just up and die. Sounds like a misfire. It happens bad when it's cold, like, I can't walk away from the bike, and I need to keep giving it throttle until it warms up a little. I have been incrementally increasing pilot jet size, and it seems to help, but I'm at a 32.5 pilot jet now and I'm wondering if I'm beginning to overjet. I have a shell cam set at .006 intake and .012 exhaust, timing is spot on, and the timing chain is adjusted.

I'm using BS38s (76-77), and right now it has a 32.5 pilot jet, and a 132.5 main, needle dropped one slot, mix screws out 1.5 turns. I have JBM diaphragms and manifolds and all genuine mikuni jets. I dropped the floats to ab 23mm.

I'll also notice this strange "burp" on decel when I'm downshifting. It's unnerving bc it almost sounds like something's hanging up.

Aside from that, power is inconsistent. If I gradually roll the throttle it will scream, but if I mash a big handful, it kind of chugs and takes a while to get going.

I feel like maybe I need to raise my needle back to the factory spec'd middle slot, maybe drop back to a 130 main, but in the past whenever I've done this it totally runs like crap. It seems to prefer a leaned needle with fatter jets.

It seemed like my bike ran better before I rebuilt it and I'm kind of comparing the old performance against the new. Am I comparing apple to oranges here with the 700 kit and the shell cam?

 

[5twins]

Are you running the specified 1200 RPM idle speed? If run lower, these bikes will just up and die sometimes at idle for no apparent reason. Also, at 23mm, your float levels are rather high (rich). Spec for that carb set is 25mm. That may have something to do with the "chugging". It may be flooding itself out when you grab a big handful of throttle.

 

[blackbetty]

OK, I'm going to reset my float height to 25mm. I noticed fuel on the diagnostic port plug when I pulled it so sync, so I figured the left carb at least is too rich.....idle speed is set to ~1200.

I'm just stumped trying to figure out why the bike seems to hangup in the middle of what seems like a healthy idle.

I also notice sometimes the idle will hang, do the slight miss, and when it regains it's at it's normal idle. So I can be decelerating from a decent speed, and when I slow to stop, idle's at about 1800rpm, does the slight hiccup, and when it falls back into it's idle it's immediately at 1200rpm. What in the hell is causing this! It's driving me nuts!

 

[jay760]

A lot of people seem to be able to get their modified engines to run on standard carbs, never been able to do it myself, always found slide carbs run much better.

 

[blackbetty]

Jay, I swear I must be a glutton for abuse. Something about the old BS38's makes me want to hang on to them. A ton of support and I really appreciate the linked setup. I also have spent so much time trying to figure them out I'd feel like a quitter if I didn't get the bike running right!

 

[5twins]

I don't see what difference a couple of CCs would make. The '73-'74 TX750 twin Yamaha used to make used basically the same carbs as the 650 of that time. In fact, they were even jetted the same.

 

[Take Warning15]

I'm having about the same problem now. I just finished my rebuild and I cannot seem to get the bike to idle at a consistent speed. Every time I come to a stop it is either above 2000 or around 1000. Half the time it just dies as soon as I come to a stop. Any advice?

 

[Jaydela180]

we should start a support group because im basically in the same boat.

as signature shows, i also am dealing with a newly rebuilt 700 motor. and having just about the same issues. heres what ive done to IMPROVE, not fix everything.

initally hard starts were result of spark gap being to large. after some research, i gathered that an increase in compression from a stock motor should be followed by decreasing gap. might help "misfire" issue.

also suffereing from midrange bog. been jetting based on the assumption that it was a rich mix. but plugs (autolite 63) have been showing super lean. after seeing punkskalar runs bp83s with his 700 motor, i switched. first, to get a better look at fuel/air mix and second to handle any extra heat. higher compression, higher heat, more problems likely?

had slight pop on decel as well, which supported lean plugs but contradicted jetting. currently up 3 from pilot stock and that should be more than enough for mods. i richened mix screw same time as changing/gapping plugs and it is now gone. am i rich? am i lean? pop is gone though.

if any of this helps, great. if not, let me know what eventually helped yours. like i said, these things improved matters, not fixed.

 

[Take Warning15]

I did just play with the idle screw. It idles better, at least for the minute or so that I let it. However, I do get a lot of popping out of the exhaust on decel. Plugs look perfect.

I had a similar problem to yours previously. Plugs were black but I had signs of lean from the engine. Turned out to be weak spark.

 

[TwoManyXS1Bs]

Hey guys, I chased the same thing long ago on my BS38s. Found 2 things. First was bad needle jet o-rings. Second, caused by the bad needle jet o-rings, previous owner(s) would torque down on the bowl screws, apparently trying to get the bowls to force-seat the needle jets, and to get the bowls to seat. Result was the carb body corners, where the bowl screws thread in, were bent down slightly, preventing the bowls from properly seating in the critical center section, where either extra air would slip in, or extra fuel would suck in (when jostled). Yes, it was maddening because idle mix, plug color and pilot jet size all seemed to contradict. Mine were pretty severe, quite noticable, when you knew what to look for.

Remove the bowl gaskets, place a dollip of grease on the middle area on the bowl, between the main and pilot jet zones, place the bowl on the carb body, remove, check if grease dollip squeezed out.

I had to place the carb body(s), upside down, in a padded vice, and very gently tapped, with a 5/16" flat tip brass punch directly over the threaded bowl screw holes (all 4 corners), until the carb body was almost perfectly flat. Then rechecked with the grease until I felt that the gasket would be able to seal that inner area.

After this fix, it idled and responded like new.

Sure hope this will fix your problem, would like to know what y'all find...

 

[XSLeo]

The size of the engine makes little difference in the carb jetting. When I did the 750 kit on my 75 I spent two weeks fooling with the carbs because I thought the bigger engine needed different jetting. It didn't. I'm right where I was as a 650.
That's one up on the pilots and two up on the mains.
What air filter are you using? The K&N pleated element and the copies of them don't work well with our CV carbs. The long straight are slightly better than the short oval or tapered filters.
Using a straight foam filter works much better.
Leo

 

[Jaydela180]

Definitely something for the bs38 guys to check. As for good old me with 34's, the second issue doesn't really apply.

Im playing with float levels right now. Ill report back with results. Thanks