Topic: Full throttle bog problem on a 76 CB750f. What else should I check

[shark]

 The problem: Give the bike any more than 1/2 throttle, the bike will bog down (if you have ever driven a GM vehicle with a Rochester Quadrajet that bogged when you floored it, but ran fine on 1/2 pedal, this is what it seems like). Bike starts easily, idles very smooth, has good throttle response up to 1/2 throttle. At more than 1/2 throttle will bog down and not accelerate. Close throttle down to 1/2 way , the bike will take off hard (I almost fell of the bike the first time, was prepared for it the subsequent times).

History:Last year was the first year the bike ran the whole summer without a major breakdown(drove 150 miles) this century. In 2008 surface rust went through the in tank and external fuel filters and clogged up the carbs. Would only drive for a few miles before it  stopped running on all cyls. Leave sit for a while and it would run again for a few miles. Took care of the rust in the tank, coated the tank, replaced both the in tank and external fuel filters, flushed out the fuel lines/removed and cleaned float bowls out on all carbs. For what it's worth, bike also has voltage drop problem even after cleaning all connections and key switch. I am using a relay to send battery voltage to voltage regulator so the battery is charging OK.

Recent work done: Clean and re-gap spark plugs (spitfire's-- know most people think they are junk, but, the bike had major starting issues until I installed them), adjust valves, adjust cam chain, install new OEM points assembly (actually, looking at the package, they might be as old as the bike), adjusted the points and timing statically and fine tuning with dwell meter and timing light (dwell on both points are between 46-49 deg with meter on 4 cyl scale-hard to give exact measurement due to meter, but both points are the same).  Centrifugal advance was checked and working. Air filter a few years old with minimal milage on it. Air box to carb boots/hoses(whatever they are called) were not on the carbs all the way, I believe they are on properly now. Didn't think to check to see if there is any air leakage at the boots until now. Fuel line seems to be routed OK. Fuel filters don't have many miles on them . Gas is from last year (with fuel stabilizer in it) but bike did the same thing with fresh gas. Air box is stock. Filter is stock type.

The question: What else should I check before tearing into the carburetors. 


Note: If I am at the point where everything points to a carb rebuild, I may do nothing this summer due to problems with my daily driver, bike is now my backup transportation.

Sorry for long post, tried to give out as much information as I could think of.

[Xnavylfr]

Try riding again and when it starts to bog, pull the choke closed a little bit at a time. If it takes off like a rocket your carbs are LEAN. You could move the main needle clip DOWN one position ,that will allow MORE gas in at the HIGHER RPMS>>



Xnavylfr(CHUCK)

[shark]

Will give that a shot.

[crazypj]

Sounds like partially blocked main jets and/or wrong fuel level
Could also be parially blocked emulsion tube holes.
It's real easy to think they are good but oxidation inside the holes reduces the actual diameter
I use a x10 jewelers loupe to check things
I use copper wire to clean things out as it is softer than brass jet so there is no chance of enlarging sizes

[ekpent]

Did you change condensors along with the points ? Sold a 75F to a friend that had issues like that and he reported back its rnning fine after condensor change.

[bollingball]

The problem: Give the bike any more than 1/2 throttle, the bike will bog down (if you have ever driven a GM vehicle with a Rochester Quadrajet that bogged when you floored it, but ran fine on 1/2 pedal, this is what it seems like). Bike starts easily, idles very smooth, has good throttle response up to 1/2 throttle. At more than 1/2 throttle will bog down and not accelerate. Close throttle down to 1/2 way , the bike will take off hard (I almost fell of the bike the first time, was prepared for it the subsequent times).

History:Last year was the first year the bike ran the whole summer without a major breakdown(drove 150 miles) this century. In 2008 surface rust went through the in tank and external fuel filters and clogged up the carbs. Would only drive for a few miles before it  stopped running on all cyls. Leave sit for a while and it would run again for a few miles. Took care of the rust in the tank, coated the tank, replaced both the in tank and external fuel filters, flushed out the fuel lines/removed and cleaned float bowls out on all carbs. For what it's worth, bike also has voltage drop problem even after cleaning all connections and key switch. I am using a relay to send battery voltage to voltage regulator so the battery is charging OK.

Recent work done: Clean and re-gap spark plugs (spitfire's-- know most people think they are junk, but, the bike had major starting issues until I installed them), adjust valves, adjust cam chain, install new OEM points assembly (actually, looking at the package, they might be as old as the bike), adjusted the points and timing statically and fine tuning with dwell meter and timing light (dwell on both points are between 46-49 deg with meter on 4 cyl scale-hard to give exact measurement due to meter, but both points are the same).  Centrifugal advance was checked and working. Air filter a few years old with minimal milage on it. Air box to carb boots/hoses(whatever they are called) were not on the carbs all the way, I believe they are on properly now. Didn't think to check to see if there is any air leakage at the boots until now. Fuel line seems to be routed OK. Fuel filters don't have many miles on them . Gas is from last year (with fuel stabilizer in it) but bike did the same thing with fresh gas. Air box is stock. Filter is stock type.

The question: What else should I check before tearing into the carburetors. 


Note: If I am at the point where everything points to a carb rebuild, I may do nothing this summer due to problems with my daily driver, bike is now my backup transportation.

Sorry for long post, tried to give out as much information as I could think of.

There are a lot of maybe here check or change every thing in red. Get a new air filter put the gas in the car or mower and get fresh gas for the bike. You will more than likely going back into the carbs.

Ken

[lucky]

You are right to get all issues resolved before working on the carbs.

I would still need to know what exhausts and what intakes you are using.

If you give it quick throttle and it bogs or quits it is lean from 1/4-3/4 throttle.
That is the range that is used most of the time when riding.
You said it idles.

[shark]

Did you change condensors along with the points ? Sold a 75F to a friend that had issues like that and he reported back its rnning fine after condensor change.

Points assy. came with new condensers installed. They are oem parts.

After installation, bike started easier and sounded much smoother before fine tuning the dwell and timing.

[shark]

The problem: Give the bike any more than 1/2 throttle, the bike will bog down (if you have ever driven a GM vehicle with a Rochester Quadrajet that bogged when you floored it, but ran fine on 1/2 pedal, this is what it seems like). Bike starts easily, idles very smooth, has good throttle response up to 1/2 throttle. At more than 1/2 throttle will bog down and not accelerate. Close throttle down to 1/2 way , the bike will take off hard (I almost fell of the bike the first time, was prepared for it the subsequent times).

History:Last year was the first year the bike ran the whole summer without a major breakdown(drove 150 miles) this century. In 2008 surface rust went through the in tank and external fuel filters and clogged up the carbs. Would only drive for a few miles before it  stopped running on all cyls. Leave sit for a while and it would run again for a few miles. Took care of the rust in the tank, coated the tank, replaced both the in tank and external fuel filters, flushed out the fuel lines/removed and cleaned float bowls out on all carbs. For what it's worth, bike also has voltage drop problem even after cleaning all connections and key switch. I am using a relay to send battery voltage to voltage regulator so the battery is charging OK.

Recent work done: Clean and re-gap spark plugs (spitfire's-- know most people think they are junk, but, the bike had major starting issues until I installed them), adjust valves, adjust cam chain, install new OEM points assembly (actually, looking at the package, they might be as old as the bike), adjusted the points and timing statically and fine tuning with dwell meter and timing light (dwell on both points are between 46-49 deg with meter on 4 cyl scale-hard to give exact measurement due to meter, but both points are the same).  Centrifugal advance was checked and working. Air filter a few years old with minimal milage on it. Air box to carb boots/hoses(whatever they are called) were not on the carbs all the way, I believe they are on properly now. Didn't think to check to see if there is any air leakage at the boots until now. Fuel line seems to be routed OK. Fuel filters don't have many miles on them . Gas is from last year (with fuel stabilizer in it) but bike did the same thing with fresh gas. Air box is stock. Filter is stock type.

The question: What else should I check before tearing into the carburetors. 


Note: If I am at the point where everything points to a carb rebuild, I may do nothing this summer due to problems with my daily driver, bike is now my backup transportation.

Sorry for long post, tried to give out as much information as I could think of.

There are a lot of maybe here check or change every thing in red. Get a new air filter put the gas in the car or mower and get fresh gas for the bike. You will more than likely going back into the carbs.

Ken

Dummy forgot to order air filter when he ordered points assy. Will get new one.

Hoses between airbox and carbs weren't on the carbs all the way. There was a air gap at the bottom. There is no more air gap, but I will have to look at some pictures to make sure I got them on all the way.

 At some point in the bikes life, a inline fuel filter was added. Will get some new fuel line to eliminate the filter just in case the filter is causing a problem. Once again will have to look at some pictures.

Will put fresh gas in bike.

The sentence about the relay was in green. Should that have been in red?

[shark]

You are right to get all issues resolved before working on the carbs.

I would still need to know what exhausts and what intakes you are using.

If you give it quick throttle and it bogs or quits it is lean from 1/4-3/4 throttle.
That is the range that is used most of the time when riding.
You said it idles.

Bike has stock air box and stock 4 into one exhaust.

I'm going to have to take it for a ride to see if there is any difference between quick throttle and easing the throttle open.

(Had a monkeywrench thrown at me with my sisters car breaking down, have to deal with that first).

[lucky]

You are right to get all issues resolved before working on the carbs.

I would still need to know what exhausts and what intakes you are using.

If you give it quick throttle and it bogs or quits it is lean from 1/4-3/4 throttle.
That is the range that is used most of the time when riding.
You said it idles.

Bike has stock air box and stock 4 into one exhaust.

I'm going to have to take it for a ride to see if there is any difference between quick throttle and easing the throttle open.

(Had a monkeywrench thrown at me with my sisters car breaking down, have to deal with that first).

With stock intake and aftermarket exhaust all you will need is a little richening of your mixture screws. About 1/4 turn.

[bollingball]

I have not seen a relay put on the regulator before. Nothing wrong with I guess just never seen it.
Ken

[Jerry Rxman Griffin aka MuthaF'er]

Would also be helpful to know how the carbs are jetted and how many miles on the bike's carbs.

[shark]

Update:

First the bike got 23,xxx miles on it. Never been in the carbs other that removing the float bowls and flushing out the rust from the gas tank episode.

Drained gas from tank and filled up with fresh gas. Took it for a spin. Snap the throttle wide open, bike bogged. Tried to ease the throttle all the way open, got going way to fast for a residential neighborhood and didn't get it to full throttle. Will have to try that test on the highway.

Looked at the air tubes(boots, whatever their called)  between the air cleaner and carbs. I got the outer ones on all the way, the inner ones are just on the tip of the carbs, but need to be pushed on further. I blame the beer I was drinking on that .

Tonight I will work on the tubes, and removing the inline fuel filter.

[flybox1]

Update:
Took it for a spin. Snap the throttle wide open, bike bogged. This is NOT a fuel injected bike.  it will always act this way.  Tried to ease the throttle all the way open, got going way to fast for a residential neighborhood and didn't get it to full throttle.   Sounds about right

still, you should service/maintain the carbs to ensure they are clean and attached to the airbox and engine properly.

[shark]

Update:
Took it for a spin. Snap the throttle wide open, bike bogged. This is NOT a fuel injected bike.  it will always act this way.  Tried to ease the throttle all the way open, got going way to fast for a residential neighborhood and didn't get it to full throttle.   Sounds about right

still, you should service/maintain the carbs to ensure they are clean and attached to the airbox and engine properly.

I did not know that.

I guess it's time to get it inspected and ride.

Wasn't a total waste. I can't remember the bike ever running this good.