'82 750LTD, bogging in the lower RPMs

Hey everyone, just bought an '82 750LTD and I'm looking for some help with a problem it's got.

Bike info:
1500 original miles!
Everything is dead stock, this bike has never been touched.

When I got the bike it wasn't running, so I removed the carbs and cleaned them. They were actually pretty decent inside. I removed all the jets and cleaned them, didn't take the diaphrams out or anything though. Also, where the pilot screws are supposed to be there is what looks to be metal plugs in place over the top of them, I'm not sure how to get those plugs out to mess with them. I reinstalled the carbs and got the bike started fairly easily, and it idles fine on its own. The problem happens when you first get on the throttle from idle. It'll bog pretty good, then as soon as it reaches around 3500rpm or so it clears right up and runs like a champ through the rest of the range.

I'm not positive this is a carb problem. I did notice that when I first got it running it was popping back through the intake, like the timing was off a bit. I was thinking I need to inspect the advance mechanism, make sure it isn't stuck or anything.

Also, battery voltage is good, and the charging system seems to be working good as well.

Any suggestions would be appreciated,
thanks
Sky

Welcome aboard! Do you have the airbox hooked up to the carbs? Ed

Yamaha_chop wrote: ... where the pilot screws are supposed to be there is what looks to be metal plugs in place over the top of them, I'm not sure how to get those plugs out to mess with them....

Those are non-tamper plugs mandated by the EPA to keep us reprobate tree-haters from enriching the ultra-lean pilot circuits.

While keeping a close eye out for a team of U.S. marshals rappelling down from a squadron of black helicopters, carefully drill a shallow pilot hole into the center of each plug, and then use a sheet metal screw to pull out the plug and and toss it at the marshals.

The cautions about carefulness and shallowness are to prevent damaging the pilot adjustment screw which is located directly underneath the plug.

Good Fortune!

650ed wrote: Welcome aboard! Do you have the airbox hooked up to the carbs? Ed

Thanks for the welcome! Yes the airbox is hooked up, everything is as it was when it came from the factory. This bike is incredibly clean, looks like it was garage kept its whole life.

Patton wrote: While keeping a close eye out for a team of U.S. marshals rappelling down from a squadron of black helicopters.

I thought I saw Tommy Lee Jones slinking around my shop yesterday!

Thanks for the advice guys. I'm unsure right now if I should mess with those settings seems how nothing has been changed from stock you wouldn't think I'd have to change anything.

Also, just because, here's a couple of my other bikes:




Both of them were originally XS650's, the chopper is a '79 and the sportbike was a '81
chopper is running a 240 rear tire, sportbike has been changed to fuel injection and is turbocharged. I'm using megasquirt engine management to control it. The sportbike isn't finished yet, hopefully a couple more months on that one. If anyone is interested I can post the build link to either bike.

I agree that since everything is in the stock config you shouldn't need to fiddle with the carb settings, jetting, etc. As you mentioned, I would check to ensure the timing advancer is working smoothly. If all is ok there you may need to make sure the pilot circuit is super squeaky clean. That is what controls the air/fuel ratio up to around 1/4 throttle. Ed

Just some thoughts/suggestions in addition the what 650ed said

I'm assuming its a 4 cylinder.
Your mention of popping through the intake at low rpms leads me to suggest checking the carb boots (black rubber connections to engine) for cracks or leaks. I had to use permatex#1 to seal mine to the engine. They should not normally require sealer but that was what it took to stop the low speed intake pops. Also check any carb vacuum caps or if you have that mess of tubing between the carbs and the cylinder head (we never had that stuff in Canada) check for air leaks or remove it altogether - do a search here.

Are your carbs Keihin (stamped on #1 carb) or Mikuni's ? If Keihin then a blockage in the transition holes will cause the motor to stutter and miss at 2-3,000 rpm. Transition holes are small and easy to miss. look into that end of the carb which connects to the engine. Open the throttle butterfly/flap. Look in and up. The transition holes are 3 small holes just where the throttle flap closes against the carb body. If the carbs are Mikunis then sorry - I have no idea if Mikunis have transition holes.

Good luck ...........and how about some pics of your almost new 750 ?

PS Do either of those 2 bikes have rear brakes ?

Thanks for the tips. The carbs are indeed Keihins. I'll check that out tomorrow.

The chop doesn't have a rear brake at all. I'm running dual discs from an '08 road glide and dual 4 piston calipers from a zx6r 636. It stops well

The sportbike does have a rear brake now, just hadn't set it up yet in that picture.

I do need to get some pics of the 750 soon.

update: I just checked the timing last night. It's spot on and the advancer unit looks very clean and in good working order.

Patton wrote: drill a shallow pilot hole into the center of each plug, and then use a sheet metal screw to pull out the plug and and toss it at the marshals.

This worked perfectly! thanks for the advice.
Cylinders 2,3,4 pilot screws came out easily enough. But cylinder 1 the screw was stuck pretty good so I have no idea what the setting was.

as for the other three:

cyl 2: 2 1/2 turns out
Cyl 3: 3 turns out
Cyl 4: 2 1/2 turns out

I am positive these have never been messed with before because the caps did not look like they've ever been removed, however these settings seem off? I thought I read in a manual I found online that they should be at around 2 turns out?

Just a quick update. I got the carbs back together and back in the bike. Messed with the pilot screws for a bit and changed out the fouled spark plugs, boom she runs great! There's a benefit to keeping the bike stock, as it runs clean throughout the rev range.

My only concern at this point is a cold start concern. The #1,2 cylinders have a hard time firing when the bike is cold. As soon as the bike warms up a bit they kick in and it runs perfectly. But I'm definitely concerned about fouling the plugs on those cylinders. I may just need to mess with the pilot screw settings on those two cylinders some more. Am I correct in thinking that the bike needs to be fulled warmed up before I mess with them though right?

Thanks again everyone for your input. Pictures to come tomorrow hopefully.

What spark plugs are you running? Ed

Yamaha_chop wrote: ... My only concern at this point is a cold start concern. The #1,2 cylinders have a hard time firing when the bike is cold. As soon as the bike warms up a bit they kick in and it runs perfectly. But I'm definitely concerned about fouling the plugs on those cylinders. I may just need to mess with the pilot screw settings on those two cylinders some more....

Enrichment (choke) circuits in carbs #1 and 2 may be failing due to blockage.

Yamaha_chop wrote: .............. But I'm definitely concerned about fouling the plugs on those cylinders. .............

Fouled plugs as in black and sooty ? to rich ? and hard to start on 1 and 2 .

There is no enricher circuit on those carbs but check if the pilot jets are blocked and check if your choke flaps/butterflies are working on 1 and 2. I've had it happen that the linkage was not operating correctly.

If 1 and 2 are rich and the mixture screws have never been messed with then I would suggest checking the fuel level in the carb bowls - may be to high. Do a search for clear tube method. Patton has great diagrams for this. Having the fuel level in spec is critical. Messing with the mixture screws will not make that big a difference regarding plug color