Backfire

[Joe shag]

Hi, new to the site and glad to have found it,
I have a 1980 cb750f just purchased it . It backfires and spits when trying to get over 5000 rpms, I had the carbs ultrasonic cleaned and checked out. So looking for the next thing to try I have no history of the bike.

Thanks

[Re-run]

Pulled the late night shift huh!
Anyways, it could be a couple things.
Do you have the stock airbox on the bike or pods? Pods work poorly on your year of bike, the carbs just do not like em. You CAN get them to work but it is not easy. The airbox, even with a K&N filter works much better.
The other option would be something in the carbs. There are diaphragms in the carbs which could have cracks. Hell it could even be something as simple as dirty jets. I know they were ultra sonic cleaned but that does not always get the jets clean. It could even just be that the air fuel mix is too lean or too rich.

If you pull your plugs and check, that will start to point you in the right direction.

[rich]

Hi Joe: Congrats on the CB750F. I was just eying one on Craig s List. Sharp looking bikes.

Is your battery fully charged and is your alternator putting out adequate voltage? It should be a bit over 14 volts at the battery with the engine running over 3000rpm. These bikes run like crap if they're not getting enough power through the coils. I just got a 750K and the battery wasn't charging. I charged it enough to start it and it backfired, missed and popped at higher RPMs. It ran a lot better after giving the battery a full charge, but only for a few minutes until the battery started to get low again. If your alternator is putting out enough, check all your connections to the ignition system and make sure everything's clean and tight.

[Joe shag]

Thanks guys I just figured out that I had responses. Still learning..well the carbs were supposed to be fully cleaned including the jets,I will look into the stock box as it does have pods. Next the charging system.

Thanks

[Re-run]

If you are just starting out, you want the airbox. Got one, and either get a stock honda filter or a K&N filter.
I doubt your idle jets have been changed but make sure the mains are stock. The number should be stamped on the jet so you can see it without removing it.
Always best to get the bike working right so you can learn it before modifying it.

[Joe shag]

So it's runing much better now couple questions yet

How do you sinc the carbs because there are no ports to hook th evacuum gauge too?.
It runs perfect up to 7000 rpms then goes flat it will work past that point by feathering the throttle no backfiring or anything.

Thanks

[sgtslag]

Check the Factory Service Manual (FSM; link on home page of this site) for instructions on how to connect the vacuum gauges to the carbs.

With regards to the high end issues, it might be caused by the weak ignition system (mainly the coils).  Check the coils' resistance (as per the FSM).  Check the wiring connections on the coils; check the plug wires and end caps for solid connections.

One of the easiest ways to improve the ignition system, is to replace the plugs with NGK Iridium DR8EIX plugs:  around $8 per plug, but they have a lower resistance across the gap, which means they fire almost perfectly, every single time.  I installed them in my '79, and it went from grinding the starter 4-5 seconds, to instant running -- the first time I hit the starter, I jumped, literally, as it was running the instant the switch closed; my idle smoothed out, and she runs perfectly.  Not only all of that, but the Iridiums will last for 50k+ miles!

There are GM coil modifications which can be done (replace the weak, OEM coils, with a standard GM car coil pair), but that is more challenging than new Iridium plugs.  The NGK Iridium plugs can be purchased at any automotive parts store:   http://www.ngk.com/results_appOther.asp?otherMotiveID=225193&mfid=1.  Cheers!

[Joe shag]

Thanks I will look for some plugs in the morning!

[shinyribs]

I would love to know more about the GM coil conversions.   Im not asking you to post it here as i know that is alot of work.But is there a site that details it?Thanks

[TOOLS1]

Pinhead just put up a post about the conversion about a month ago. Was very detailed. I saw his bike with the conversion done. Ran, and sounded great.
TOOLS

[TOOLS1]

To sync the carbs, you remove the pan-head screws from the carbs near the rubber intake boots. Then install the fittings that come with the gauges.
TOOLS

[Joe shag]

That's the problem..no screws to remove???

[TOOLS1]

My bad On your bike the screws are on the head. Right by the rubber boots.
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[Joe shag]

Nice ..they are there thank you very much!

[Joe shag]

Ok so she's running pretty good now smooth up through the full rpm range but I don't think I am charging the battery, it does have a new stator so need to diagnose the system and see why ? New battery should have been my clue when I purchased the bike..any help appreciated