Topic: Mystery miss 75 750F (fixed)

[Don R]

OK I tried to fix this without posting but I'm stuck. It's a 75 F 750 with 8500 miles. It runs ragged (lousey) at cruising speed, if I get up around 70 it smooths out, pulls OK under load. I've checked- No strange noises from engine, Exhaust not plugged, fresh fuel and oil change, good fuel flow, stock new air filter and box, stock header but custom muffler, stock jets, middle needle position, cleaned carbs and set floats several times, re-checked float level with tube, slides installed properly, Manifold rubbers look good and I find no vac leak, static and running timing OK, advance OK,  points look good, gap OK, no significant sparking at points, 28 to 30,000 volts at plugs, I trimmed the plug wires and re-assembled, valve action looks OK, lash good, a little loose, plugs look rich. Compression good and equal, #4 pipe a little blue, they all get hot enough to toast your hand. New fuel line not kinked, coils wired properly, starts good with a little choke, idles fine. It was very rich then I discovered I could remove the idle screw covers and the screws were way out, I adjusted them to 7/8 out and it got somewhat better. Good battery and charging,
 My next move is to try another set of carbs, I have a good set but I robbed the floats for my other bike so I need new floats.
 Anything I missed? Another new set of plugs? Maybe I'll drop the air filter and try it again.

[Jerry Rxman Griffin aka MuthaF'er]

Don,

What's your plug chop look like at mid range? Custom muffler and stock middle position on needle? I never blued my stock pipe (?) even after I did the 812 and didn't understand jetting. Surprised at that  But just on one? Valve lash a little loose? If they aren't opening enough you may not be getting enough air. Sounds like you're close.

[Don R]

It's been so rich I'm a little gun shy about moving the needle. this is the first time I've needed the choke to start it. That might mean I am closer on the carbs. The pipe was blue when I got it, no telling what a PO may have done but I can't find anything other than the carb screws being so far out. I need to get some new plugs and do a chop, last fall I did one without the filter in and it looked the best I had seen.
 Maybe it's been fat down low and lean at higher rpm's. The lash is very close, just not as snug on the feeler as I usually get. I think it's all factory stock so no big cam.
 Thanks for the feedback, that's what I'm looking for.

[Steve F]

You said you trimmed the plug wires?  I'm assuming that you trimmed them at the plug end.  If you tried to trim them on the coil end, how did you reattach them?  Unless you have aftermarket coils, it's not easy to get them to work right again on the stock coils since they're epoxied in place.  It sounds to me that the coils and/or wires are the culprit. 
If I'm not mistaken, the adjusting screws are only effective on the idle circuit and the main jets take over at around 2 to 3k rpm.  Remove the emulsion tubes and the main jets and check that the tubes' cross-drilled holes are ALL clear and no build up of crud inside.  I know it's hard to check fuel flow while riding, and it is possible that while you have a good "static" fuel flow while the engine is off, you "could" be starving the engine at higher rpm's...maybe a blockage in the fuel tank cap which creates a vacuum in the tank?  Just a thought.
Steve F

[Jerry Rxman Griffin aka MuthaF'er]

Don,

One other thought. I once rebuilt the carbs with Keyster stuff. After which I couldn't keep a set of plugs from fouling around town with very little riding. I assumed since I'd put new carb parts in that couldn't be the problem. I'd have to get the R's up to keep the plugs cleaned. I reinstalled the original parts and the problem went away. Turns out the idle screws were not correct. I forget which way but one set had holes in the tips and the other didn't. Plus the needles were rough finished, not smooth with the turning marks obvious which I didn't like.     

[w1sa]

It sounds like it's still running rich on the low speed circuit.   I'd adjust on the lean side  to the point it requires full choke to start and when warmed up still responds well to off idle throttle opening. At low speed cruise (up to say 55-60) at fairly constant throttle I'd be looking to see clean plugs with brown/ majenta colour.

At 70 you've probably transitioned more onto the mains/needle and are within a more acceptable fuel/air burn ratio for sustained throttle position.

Has it had a vac sync on the carbs?

Just my thoughts on what you've said

I

[Don R]

Bench sync, alas, the mercury was missing from the balance tube, glad it wasn't here at the time. Yes, I trimmed the plug end of the wires per recommendation here. although they look good I wish I could install new wires, the emulsion tubes are clear including the thru holes. I blew out all passages too. All the parts in the carbs appear to be genuine Honda. I will check the tank vent for a lap around the block.
  I do have other coils to try but the wires are awful looking. I did check volts at the plug.
 Thanks and keep em coming I'm willing to try em all.

  I just tuned up my wifes car and it ran the same when I was done, cleaned injectors, intake gaskets. Spent some money on it. I had bought a bad plug wire set, #5 had a pinhole in the cable and lit up the carb cleaner while checking for an intake leak. My buddy lost some hair and eyebrows. Yeoow!

[Don R]

Back to carbs, I can turn off the fuel and it cleans up and runs nice till the fuel is gone, then give it a shot of fuel, runs good, leave the petcock on, the miss comes back. funny, the fuel doesn't overflow and I checked the float level (last fall) with a clear tube on a modified float bowl.

[Don R]

OK, it runs good with the air filter off! It is  stock with a new filter. 105 jets and stock needles. I ordered new honda jets in case they have been drilled by a po. Plugs still look black. maybe I'll need pods to make it run.

[Don R]

My co-horts say lose air filter and bike runs better= vacuum leak. What say the forum?

[Spanner 1]

Yep, drilled main jets!... I think your jets were drilled to 'match' a 4 into 4 drag pipe set-up so common in the 80's and never fixed....
put the right jets in 'er and all will be well !...

[Don R]

Yes, it was drilled jets. Not the mains but the low speed jets. It runs great now. I wonder how long ago the PO drilled them and if they ran it much like that. They probably did it when the stock muffler went away. Compression is good and equal so I think I will de-carbon it and buy some plates!

Oh yes, thank you for your ideas and suggestions.