won’t rev past 5000 backfires when I close the thottle

Posted: Thu Jun 20, 2024 1:26 am

Hi guys, new to the forum! Old CB 750 C. Clean the carbs as best I could, (I’ve downloaded Sean‘s carb manual and the FSM)

170 psi compression plus or -10 across-the-board

Lowered Gap on the inductive pickups left side to lowest feasible limit

Only 10.5 V to coils so I jumped them directly to battery which also gave the TCI modules proper voltage

Sparks still seemed erratic to me so I checked coils caps and wires and found two caps reading 400k and 330k ohms vice their spec 5k ohms!!!
Coils checked at 2.7 ohms and 20 k ohms for both on the dot. Wires were good after trimming the ends

BUT! after taking the caps apart cleaning the springs ,resistor ends, and terminals and reassembling them and having them recheck at 5K ohms!

Still have the same problem!

Am I left with the TCI modules? Or revisit the carbs?

John

Posted: Thu Jun 20, 2024 8:21 am

Welcome to the site John ... Smile

If you haven't read the following page , now is the time .

http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&p=80601#80601

If you run into problems with any of that , Please let us know . Thanks . Smile

Posted: Thu Jun 20, 2024 10:49 am

Welcome here wave

Won't rev over 5000 is often due to weak charging. You should have 14.4 VDC measured at the battery at 3000 RPM on a full battery.

The backfiring could be from a header leak or leaky intake at the insulators. Leaky intake also can cause floating idle.

Posted: Mon Jun 24, 2024 1:12 am

14.4 v at even 2000rpm with new battery

New intake boots on rebuild since they were rock hard

Digging into new member thread

Posted: Mon Jun 24, 2024 2:08 am

Make sure the carbs are fully seated in the insulators. It's a common issue that they don't get fitted all the way in, then leak.

Frank wrote:

Make sure that you have the carbs fully seated, the rail that holds the bank together should be touching the insulators (rubber boots that go between the carbs and head)

On a roll Joined: May 03, 2024 Posts: 104 Location: Canada

Clean the carbs as best I could,

You don't sound too convincing. Tell us more about how you cleaned the carbs. Aside from the carbs, when was the last time the valve clearances were checked?

Posted: Thu Jun 27, 2024 1:24 am

FLAN wrote:

Clean the carbs as best I could,

You don't sound too convincing. Tell us more about how you cleaned the carbs. Aside from the carbs, when was the last time the valve clearances were checked?

Indeed. These carbs are particularly hard to clean, and also tend to run poorly when the least bit dirty. Ultrasonic washes them best. Careful of seals and gaskets on these old carbs, avoid strong chems.

These bikes also like to burn valves if they get too tight, but your compression is good. I recommend .005" ±.001" valve clearance.

Posted: Sat Jun 29, 2024 3:44 am

The compression was so good I didn’t bother checking valve clearances but I will do that. I will also pull the carbs and And put them in my ultrasonic cleaner as I did not do that the first time.

Seems to be a ton of emphasis on carburetors for these bikes but is there anything ignition wise I can check other than what I checked already

Posted: Sat Jun 29, 2024 3:56 am

I did not bother with the valve since the compression came out so good but I will do so

Looks like I will be pulling the carbs off and soaking them in the ultrasonic cleaner but all the passages sprayed through correctly after cleaning

There seems to be a lot of emphasis on carburetors on this website for these bikes. Is there anything additionally I can check on the ignition side of things?

Posted: Sat Jun 29, 2024 3:58 am

Sorry about the double posting, I just wanted to make sure that my signature showed up after reading the new member thread

John

Posted: Sat Jun 29, 2024 4:08 am

Thx frank on info about carb assembly rail touching insulators

On a roll Joined: May 03, 2024 Posts: 104 Location: Canada

There are a few reasons for the carb problems on these old bikes. Today's gas is not what it used to be and when these carbs sit with old gas left in them problems arise. Be sure to separate the carbs and fully disassemble them. Once you think you have them clean enough then do them again. Use quality carb kits, not the junk on Amazon. Install a fuel filter. As for your valve adjustment, they need to be checked. Don't assume they are good because you have good compression. You might be one ride away from that all changing.

Posted: Sat Jun 29, 2024 4:51 pm

Ltbordo wrote:

...There seems to be a lot of emphasis on carburetors on this website for these bikes. Is there anything additionally I can check on the ignition side of things?

Our charging systems and ignition systems have always been problematic on the DOHCs. Check my New (to you) Bike Checklist for the things to look for. It'd be worth verifying secondary resistance on each coil to make sure one of the plug boots hasn't gone out of spec (quick check from one sparkplug boot on #1 to sparkplug boot on #4 and compare to #2 & #3). IIRC, the secondary of the coil S/B 12K - add in the 5K per boot and you get 22KΩ (someone will correct me on the coil secondary resistance, if I have that wrong). But the quick way of comparing 1/4 against 2/3 is likely to be good enough, irrespective of the absolute readings. Alot of folks pull the plug wires from the coil, trim off 1/4" and reinstall (after checking the condition of the threads on the coils' screw tips they screw into), and do the same on the boot end. Cracked coil cases are common since they sit above the hot cylinders, and any moisture getting into those cracks cause shorting of the winding, and reduce the effectiveness of the spark.

The New (to you) Bike Checklist goes over everything else (pulser gap, coil voltage, charging voltage).

Yes, definitely validate your valve lash first, tho. A lot of electrical problems end up getting blamed on the carbs, since there's 4 of them, and "they're complicated!"...

Posted: Sun Jun 30, 2024 1:22 am

Well, I’ve got my marching orders

Valve clearance.

Completed seat carbs/recheck for vacuum leaks

Re comb over ignition system and check to see if the spark disappears at 5000 rpm with 2 gap checkers installed (one on number 3 cyl and one on number 4 cyl)
If the spark stays strong at that RPM but the motor bogs down and back fires off throttle…..

off come the carbs again

when I get back from vacation I’ll reattack the problems and keep you guys posted

Posted: Sun Jun 30, 2024 1:30 am

Thx Sean for that carburetor guide

it is a work of art!

Posted: Sun Jun 30, 2024 1:52 am

I’m also going to pay particular attention to voltage supply to the coils and TCM modules at that 5000 RPM mark After rechecking all the coil wire and resistor cap resistance values

At 5000 RPM I should be on the main jet circuit which is the simplest circuit on any carburetor (On a lot of carburetors it’s the only circuit!)

And even on two carburetors on their main jets I should be able to rev over 5000 RPM no load?

Wish me luck Zen Buddha Masters of this forum!

Posted: Sun Jun 30, 2024 3:16 pm

Which jet circuit you're on is predicated by intake vacuum (on cv carbs) and throttle position, not engine speed.

Posted: Mon Jul 01, 2024 1:36 am

I assumed at half throttle I’d be on main jet on these carbs. Even at that throttle steady state with no load would be pretty high vacuum…..

Posted: Mon Jul 01, 2024 12:13 pm

Twisting the throttle only opens the butterfly. It's the combination of load and throttle position that causes the vacuum to change and change the slide position.
Since these slides are operated by vacuum, it is difficult to know for certain how far it's actually open. Placing a camera can help.

Here's Melchiro's (from the F site) video of DOHC carb inlet and slide with a v.stack. You can see around 2:10 that when just cruising, the slides are barely open. It's only under hard acceleration that they really open up.

So, at 5000 rpm, with no load, the slides will barely be open. If you're under heavy load, like full throttle acceleration up a hill at 5000 rpm, the slides could be wide open. WHich jet circuit is predominate at any given time depends on the slide position.

Posted: Mon Jul 01, 2024 3:07 pm

genesound wrote:

...You can see around 2:10 that when just cruising, the slides are barely open. It's only under hard acceleration that they really open up.

So, at 5000 rpm, with no load, the slides will barely be open. If you're under heavy load, like full throttle acceleration up a hill at 5000 rpm, the slides could be wide open. WHich jet circuit is predominate at any given time depends on the slide position.

And THAT's why its so critical that the slowspeed circuit is clear - it's being used constantly. One or two carbs with poorly cleaned slowspeed jets can give you some weird symptoms...

Posted: Tue Jul 02, 2024 1:16 pm

Genesound….. Not only was that video entertaining as hell to watch but quite educational

John

Posted: Fri Jul 19, 2024 4:13 pm

SOLVED!!

at least the smoking gun is found....

coil dwell recharge failing at 5000rpm (even with the wires trimmed and caps disassembled and cleaned and reading 5k ohms)

even with the coil primary and secondary resistance checked within specs

even with good voltage and grounds to both coils (and TCIs) checked

EVEN with a good spark 1/4 inch gap spark at idle!!!

checked also the pulse generators were putting out a good signal (although research suggested since they were a magnet over coil analog pulse generator and not a hall effect pick up they would deliver a STRONGER signal as RPM increased not weaker....)

SO!

checked the spark on the gap checker as the motor went to 5000rpm and the left coil spark output on 2 and 3 cylinders cut off completely and the right coil output faded at around the same RPM and went full bright as RPM was dropped below 5000RPM.

put in new coils (and new wires and caps of course) and the engine roared to life

still a smaller backfire out of number 4 which could be from a failing float needle (fuel rises till the upper float pickup drips out of the drain hose) or maybe a partially open exhaust valve.....still haven't checked valve clearances but will SOON!

found a small vacuum leak on the number 3/4 carb side with carb cleaner but cannot pinpoint it so will spend some time there over the weekend

after checking valve clearances will see if backfire goes away but I suspect carb issues due to the vacuum leak location and the aforementioned high fuel float level in number 4 carb so carbs will come off again for another slow circuit clean, needle and seat polish, SOS and air cutoffs delete

never have had coils with good resistance checks and a good spark at idle be the culprit before but.......

thanks for all the help and I'll keep you guys posted

john bordelon

Posted: Fri Jul 19, 2024 4:55 pm

OK good, getting better.

Did you look for cracks in the coil cases?

On a roll Joined: May 03, 2024 Posts: 104 Location: Canada

Good to here you are making headway. It never hurts to check valve clearances if you don't know where they are at. The vacuum leak could be causing your backfire as well.

Posted: Fri Jul 19, 2024 11:52 pm

A head/header gasket leak is worth investigating if it's a pipe afterburn type backfire on closed throttle. Those donut gaskets can be tricky to seal.