Topic: K7 no vacuum reading below 2k RPM

[Vin750]

Guys I need help solving this challenge.

I am finishing a K7 (PD41A carburettors) and it can’t get a vacuum reading at idle. I can only get it above 2.5-3k RPM. Yes the gauges work, and no there is no vacuum leak, I have sprayed wd40 everywhere and no sign of it being sucked in.

I actually connected the gauges to my other CB750 and they read 18-20 cmHg at idle.

So this bike has had valves, cam chain, timing, points gap adjusted. It’s all set correctly, or at least it should be. Valves are 2 and 3 thou as per the manual.

Carburettors have been cleaned very well, all settings are set to default apart from the needle jet clip which is on the second notch from the top. Main is 115 and slow is 35, mixture screw is 1 and 1/2 out.

Compression is good 120, 125, 130 and 125.

The bike starts easily and holds idle well but it sounds clunky like it’s out of sync. I have synced the carburettors as far as could but it still doesn’t sound right. When I rev it past 2.5k or 3k RPM I get vacuum readings.

When at 3k RPM it takes time to come back down after I close the throttle. And sometimes it races up to 3k RPM, yes it behaves like there is a vacuum leak but it can’t be that I have a generalised leak on all four carburettors.

Any ideas as to why this could be happening? I have gone over everything and can’t figure out why there is no reading at low RPM.

Thanks in advance!


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[evinrude7]

Guys I need help solving this challenge.

I am finishing a K7 (PD41A carburettors) and it can’t get a vacuum reading at idle. I can only get it above 2.5-3k RPM. Yes the gauges work, and no there is no vacuum leak, I have sprayed wd40 everywhere and no sign of it being sucked in.

I actually connected the gauges to my other CB750 and they read 18-20 cmHg at idle.

So this bike has had valves, cam chain, timing, points gap adjusted. It’s all set correctly, or at least it should be. Valves are 2 and 3 thou as per the manual.

Carburettors have been cleaned very well, all settings are set to default apart from the needle jet clip which is on the second notch from the top. Main is 115 and slow is 35, mixture screw is 1 and 1/2 out.

Compression is good 120, 125, 130 and 125.

The bike starts easily and holds idle well but it sounds clunky like it’s out of sync. I have synced the carburettors as far as could but it still doesn’t sound right. When I rev it past 2.5k or 3k RPM I get vacuum readings.

When at 3k RPM it takes time to come back down after I close the throttle. And sometimes it races up to 3k RPM, yes it behaves like there is a vacuum leak but it can’t be that I have a generalised leak on all four carburettors.

Any ideas as to why this could be happening? I have gone over everything and can’t figure out why there is no reading at low RPM.

Thanks in advance!


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have you bench synced the carbs?  after that i'd check points and timing again.  the random rev'ving up and down sounds like the carbs are just way out of sync. 

stock airbox? 

[Vin750]

Yes bench synced and stock airbox.

At this stage I am more curious about the vacuum issue.


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[evinrude7]

float heights all good?  other than that i'd say there is a vacuum leak some place.

[Vin750]

Yes it’s at 12.5 which is the default for that carburettor


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[madmtnmotors]

What is the condition of the boots between the carbs and head? Original and hard as a rock? New aftermarket boots? Somewhere in between? The original boots can become so hard that the clamps can't get them to seal.

[Vin750]

They are new aftermarket. And I have spayed so much wd40 around their to find a leak but nothing.


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[madmtnmotors]

You didn't get the boots from cruzinimage did you?

http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841

Also, I know the Honda manual indicates the float height for the 77' model at 12.5mm, but I believe that may be a misprint. AFAIK all of the CB750 PD "keyhole" carbs are the same (at least their bowl chambers and floats), and should be set to 14.5mm. This is the float height for the 78' CB750 as well as the F2 models. The Clymer manual lists the float height for the K7/K8/F2/F3 at 14.5mm as well. I'm not sure if the float height could cause this issue, but I would suggest that the K7 PD carb float heights be set to 14.5mm. This should pretty much make the flat part of the float parallel with the bowl flange.

[TwoTired]

Have you closed the dampers for the gauges too far?  What type of manometers are you using?

Seems to me that if the engine is running, it has to have vacuum to make the carbs provide a fuel air mixture.

How are you connecting to the manifolds?

[scottly]

Is the bike only running on two cylinders? If it is, a much larger throttle opening would be required to maintain an idle speed, which would reduce the vacuum. Do the spit-sizzle test on the exhaust pipes.

[Vin750]

You didn't get the boots from cruzinimage did you?

http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841

Also, I know the Honda manual indicates the float height for the 77' model at 12.5mm, but I believe that may be a misprint. AFAIK all of the CB750 PD "keyhole" carbs are the same (at least their bowl chambers and floats), and should be set to 14.5mm. This is the float height for the 78' CB750 as well as the F2 models. The Clymer manual lists the float height for the K7/K8/F2/F3 at 14.5mm as well. I'm not sure if the float height could cause this issue, but I would suggest that the K7 PD carb float heights be set to 14.5mm. This should pretty much make the flat part of the float parallel with the bowl flange.

I did get it from CI, that thread is horrifying.

And they aren’t great but i managed to install them and they seem ok for now. There cannot be a vacuum leak in all the four of them though. And if there was I would have found it.

Calling a misprint makes perfect sense though. I have thought that too but due to lack of supporting information I never really did anything about it. The pain now is removing all that to reset the height.


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[Vin750]

Is the bike only running on two cylinders? If it is, a much larger throttle opening would be required to maintain an idle speed, which would reduce the vacuum. Do the spit-sizzle test on the exhaust pipes.

Done that many times and no it’s not running on two cylinders. It idles fairly well at 1100 RPM.


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[scottly]

You can easily do the "clear tube" test on the PD carbs. Do that before taking anything apart. What do your spark plugs look like?

[Vin750]

You can easily do the "clear tube" test on the PD carbs. Do that before taking anything apart. What do your spark plugs look like?

They look dark, no oil and not wet. But then again I have been running this bike at idle forever to try and get to the bottom of this issue.

[Vin750]

You can easily do the "clear tube" test on the PD carbs. Do that before taking anything apart. What do your spark plugs look like?

I will do the clear tube test, where should the level be?


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[scottly]

About 3mm below the top of the float-bowl seam.

[madmtnmotors]

I did get it from CI, that thread is horrifying.

LOL! Indeed it is. Seeing as how they are brand new you should be good for the short term. The boots I got from Vintage CB750 seem to be holding up a little better.

Your issue is very strange. I don't recall any recent threads that describe the symptoms of vacuum leaks yet no vacuum leaks detected. Did you follow TwoTired's suggestion and check the gauge dampers to verify that that you are starting the test with the dampers wide open?

[Vin750]

Have you closed the dampers for the gauges too far?  What type of manometers are you using?

Seems to me that if the engine is running, it has to have vacuum to make the carbs provide a fuel air mixture.

How are you connecting to the manifolds?

I missed this post, I don’t have these dampers on the gauge. I have a tool called Morgan carbtune which works well and works well on all my other bikes.

I am connecting using the screw holes on the carburettors.


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[Bodi]

If it's idling, it has vacuum. The only other option - to actually have the manifold at ambient pressure - is an extremely lean mixture that almost certainly wouldn't fire.
Idling a gas engine has the engine balancing the power it gets from the low energy fuel/air charge against the effort to pull the pistons down against the manifold vacuum. Something is wrong with your measurement procedure.

[TwoTired]

Something is wrong with your measurement procedure.

Agreed.

Carbs work on the principle of differential pressure; carb throat to outside atmospheric.  If there is no vacuum in the carb throat, there is no differential pressure to drive fuel through the jets.  There is no other fuel pump to draw fuel up from the carb bowl into the engine.
Carbs don't feed fuel to the intake runners when the engine isn't running.  Neither will it feed when there is no throat vacuum.

...Unless you have a localized suspension of known physics principles...  

...Then give NASA a call.

Otherwise, show us a pic of the screw holes you've selected on the carbs.

Cheers,

[evinrude7]

You didn't get the boots from cruzinimage did you?

http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841

Also, I know the Honda manual indicates the float height for the 77' model at 12.5mm, but I believe that may be a misprint. AFAIK all of the CB750 PD "keyhole" carbs are the same (at least their bowl chambers and floats), and should be set to 14.5mm. This is the float height for the 78' CB750 as well as the F2 models. The Clymer manual lists the float height for the K7/K8/F2/F3 at 14.5mm as well. I'm not sure if the float height could cause this issue, but I would suggest that the K7 PD carb float heights be set to 14.5mm. This should pretty much make the flat part of the float parallel with the bowl flange.

I did get it from CI, that thread is horrifying.

And they aren’t great but i managed to install them and they seem ok for now. There cannot be a vacuum leak in all the four of them though. And if there was I would have found it.

Calling a misprint makes perfect sense though. I have thought that too but due to lack of supporting information I never really did anything about it. The pain now is removing all that to reset the height.


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as i understand it you don't have to have a leak on all 4 to get the symptoms you have.  someone please correct me if i'm wrong? 

[Bodi]

Unless I have badly misread something, this engine idles but there is no manifold vacuum. The manifold is everything past the slide - and each carb adapter has a vacuum measuring port. If a gasoline engine is idling, there is some manifold vacuum. So what's going on? I do not know.
One adapter leak would give poor vacuum on that cylinder only and a rough idle since that one would be firing poorly. idle speed could be OK or even high - less work on intake stroke due to the leak. But a vacuum gauge set would show that very clearly. It wouldn't lose vacuum on all four. All four leaking (badly) would give poor vacuum readings on all but in my experience getting an engine to idle with leaks serious enough to show no vacuum ... not easy.
So still no idea what is doing this. Are the spacer vacuum gauge ports drilled through to the spacer throat? Did you test the carbtune with a vacuum cleaner or whatever to confirm it works? I believe the carbtune works by having air flow past the indicator rods but I don't want to go out in the cold to find mine and look at it right now - your spacers could all be leaking and the carbtune range just isn't showing the vacuum present? Possible the carbtune is not capable of reading a vacuum below a "runnable" level? It only has to read "normal" vacuum to work fine for syncing carbs where everything is good.
Do the rods slide up for a while if you rev the engine and close the throttle?

[samm_j2]

did you ever get this figured out??

[Vin750]

did you ever get this figured out??

I haven’t, I have so much on that I haven’t been able to finish this. I will get back to it soon and sell the bike.


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[tlbranth]

I've seen the 'WD40' for leaks mentioned on several forums. If I were looking for a vacuum leak I'd use propane or acetylene.....probably propane.