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I decided to hook up another car to it and spend a good deal of time cranking it and playing with choke/throttle/idle adjuster and eventually managed to get it to idle of sorts. ANY throttle at all kills it and the choke has very little effect. I guess it's progress but I'm now left with 2 thoughts

1. What if someone put something in the fuel tank
2. Wonder if those jets are doing something funky

I'm still not sure whether the mixture is lean or rich but I'll pull a plug again when I get a chance to find out
Weird that you're having trouble with it even running now. Did you separate the carb bodies this time around? Any chance the sync got messed up?

Can you confirm that you still have good compression? I can't remember if we already did that. Also, you put the original choke plugers back in properly as well as the rail that operates them?

Typically opening the butterflies and having it die is a symptom of no fuel and all air, or it loses pressure and the venturi effect to draw the fuel out is compromised.

Keep at it, keep posting, and MORE PICTURES :wink
 

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Discussion Starter #62
Sorry for the delay, just got back on the car today, in response to the questions:

"Did you separate the carb bodies this time around? Any chance the sync got messed up?"
Nope, stayed together so no synch issues

"Can you confirm that you still have good compression?"
I did compression test it previously, all good. Still good by the looks of things

"Also, you put the original choke plugers back in properly as well as the rail that operates them?"
Yup


So, today went like this:

1. Remove carbs, change the main jets back to the originals and the secondary jets too
2. Refit the carbs and limp it back into an idle (better with the originals back in)
3. Check the CO (1.6%)
4. Use fuel from a can to rule out that
5. Fit the alternative fuel pump to rule out the lines and pump
6. Pulled a plug (looked lean)

All of that and it still died any time I gave it throttle. Must have spent 30 minutes at idle all in with choke and throttle and idle adjustments.

Finished by checking carb balance and it was perfect. Still wouldn't take any revs.



Now the weird bit - I completely covered the 4 trumpets to try and starve it of air and make it richen up. It started hunting a little bit and revving differently. I then covered each one with my hand one at a time and saw the needle lift up and it resumed to idle.

This seemed to clear its lungs and then it revs freely and everything is perfect again.

I have zero confidence to take it out now but it seems to have cured itself somehow
 

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Now the weird bit - I completely covered the 4 trumpets to try and starve it of air and make it richen up. It started hunting a little bit and revving differently. I then covered each one with my hand one at a time and saw the needle lift up and it resumed to idle.

This seemed to clear its lungs and then it revs freely and everything is perfect again.

I have zero confidence to take it out now but it seems to have cured itself somehow
Well done for the sensible sequential strategy of investigation, James. :thumb:

What do you have in this filter canister (immediately under the air box inlet)?
 

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Ha, thanks - what I have in there is the foam I found in the carb, it is the texture of chewing gum so won't be helping much.
OK. Above each Main Jet piston, there is a volume of air above the diaphragm.

Each one of those 4 volumes are interconnected and exit by a black plastic T-piece, through a rubber hose to that filter housing.

I've seen the housings empty (no filter, even though they cost a couple of quid).

I have no experience of this, but, blocking that filter housing, cannot be good for free movement of the main jet pistons.

Are you up for an experiment?:

Take the filter housing off, start the engine up and see what happens when you blip the throttle.

Then block the rubber tube and see what happens .....
 

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Ha, thanks - what I have in there is the foam I found in the carb, it is the texture of chewing gum so won't be helping much.
That is supposed to be a very light thin sponge/foam filter. That looks like it has seriously degraded after 26 years of seeing fuel and other contamination. :frown
 

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Ha, thanks - what I have in there is the foam I found in the carb, it is the texture of chewing gum so won't be helping much.
You need to replace it with this aptly named gum. Just as Eucalypt and Menthol clear the throat, it should also clear jets and fuel passages:grin
 

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Discussion Starter #69
:D

I fired it up tonight, it's cold and started perfectly on choke and idle nice revving smoothly.

I blocked the tube and blipped it and we started to get a similar symptom as before. I struggled to get it to stop once it started (I was covering the trumpets like before) but changing the idle speed made it disappear immediately.

Possibly onto something with this but not sure. I've got it back to idling/revving but I think it's a bit lumpier than it once was. I guess I need to drive it under load and see what happens, still a bit concerned I haven't found the issue but will see how I go!
 

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:D

I fired it up tonight, it's cold and started perfectly on choke and idle nice revving smoothly.

I blocked the tube and blipped it and we started to get a similar symptom as before. I struggled to get it to stop once it started (I was covering the trumpets like before) but changing the idle speed made it disappear immediately.

Possibly onto something with this but not sure. I've got it back to idling/revving but I think it's a bit lumpier than it once was. I guess I need to drive it under load and see what happens, still a bit concerned I haven't found the issue but will see how I go!
That's quite encouraging. :grin

When you stripped the carbs, did you check the diaphragms for any splits/holes?

How rubbery was the rubber? Please don't laugh, but when I strip carbs, I clean and condition the diaphragms. Making sure that they are nice and flexible. :nerd
 

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Discussion Starter #71
:D I did indeed check each one for splits, nicks and flexibility. All appeared realy good which was a nice surprise.

The weird thing is it doesn't appear to be a single cylinder that goes, whatever's happening affects all of them. I'll keep prodding and poking and report back if I progress at all
 

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:D I did indeed check each one for splits, nicks and flexibility. All appeared realy good which was a nice surprise.

The weird thing is it doesn't appear to be a single cylinder that goes, whatever's happening affects all of them. I'll keep prodding and poking and report back if I progress at all
Excellent!

Final thought:

- I am assuming that the direction of air flow through that secondary filter system is toward the engine.

- That decomposed gungey filter seems to have a bit missing ..... that would have been drawn through to above the diaphragms.

So, could that debris be (or have been) blocking the "drilling" in the main jet piston(s) that damps the movement of the pistons?

Did you "blow out" those drillings?
 

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Discussion Starter #73
I think it will be bidirectional when the diaphragm moves up and down?

See post 59, found the foam already!

Yes blew out all of those with compressed air and carb cleaner before that, if you have any other thoughts lmk!
 

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I think it will be bidirectional when the diaphragm moves up and down?

See post 59, found the foam already!

Yes blew out all of those with compressed air and carb cleaner before that, if you have any other thoughts lmk!

Aha!

Post #20 : "2 large pieces of foam melting away inside the carbs (looks like part of an air filter)".

The hint was there all along! :grin
 

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Discussion Starter #75
:( Oh man I'm running out of ideas

I took it out for a spin, ran perfectly for the first 5 minutes. Started to warm up so I gave it a few revs and spluttered a bit then cleared itself and ran well. It was revving freely to about 7-8k rpm no problem.

Then it died again. Any kind of throttle and the engine gave up completely. I managed to limp home with the idle adjuster turned up so it was at about 3k rpm and the choke out to give a bit of fuel, not a pleasant journey at all.

It was popping and banging in the exhaust if I put the clutch in but all the plugs look slightly lean if anything. I put the CO meter on it and it's all over the shop, one minute rich and the other lean.

I'm toying with the idea of a second hand set of carbs to change everything in one go
 

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The little filter is to maintain the air pressure in the float chambers i do believe.
If the carbs are not seated on the manifold you may be drawing in air. The normal symptom for that is a high idle and no power. Not a hunting idle like you described but worth a look since you are hunting around for things to look at. Fairly easy to check for with a can of aerostart or the like. If you spray where the leak is the idle will pick up.
The fact you are using the choke to richen it up means a blockage in the system somewhere. Even though you have been through all of it with a fine tooth comb it is worth another look. Possibly one or more of the floats are a bit too low. Blow all orifices with air in case of any errant bits of rotten foam.
You'll get there in the end.
 

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Discussion Starter #77
Thanks for the encouragement, I'm quite sure I seated them well but will give it a go anyway

I put a boroscope in the fuel tank but couldn't see anything obvious there
 

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:( Oh man I'm running out of ideas

I took it out for a spin, ran perfectly for the first 5 minutes. Started to warm up so I gave it a few revs and spluttered a bit then cleared itself and ran well. It was revving freely to about 7-8k rpm no problem.

Then it died again. Any kind of throttle and the engine gave up completely. I managed to limp home with the idle adjuster turned up so it was at about 3k rpm and the choke out to give a bit of fuel, not a pleasant journey at all.

It was popping and banging in the exhaust if I put the clutch in but all the plugs look slightly lean if anything. I put the CO meter on it and it's all over the shop, one minute rich and the other lean.

I'm toying with the idea of a second hand set of carbs to change everything in one go
That is very discouraging. :rant:

A second set of carbs is always a good idea. :thumb:
 

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:( Oh man I'm running out of ideas

I took it out for a spin, ran perfectly for the first 5 minutes. Started to warm up so I gave it a few revs and spluttered a bit then cleared itself and ran well. It was revving freely to about 7-8k rpm no problem.

Then it died again. Any kind of throttle and the engine gave up completely. I managed to limp home with the idle adjuster turned up so it was at about 3k rpm and the choke out to give a bit of fuel, not a pleasant journey at all.

It was popping and banging in the exhaust if I put the clutch in but all the plugs look slightly lean if anything. I put the CO meter on it and it's all over the shop, one minute rich and the other lean.

I'm toying with the idea of a second hand set of carbs to change everything in one go
I would want to see what the main jet pistons are doing if you start her up and then 1/ slowly increase the throttle from low to higher idle, and 2/ blip the throttle.

Can you make and post a video, James?
 

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Discussion Starter #80
Sorry for the delay, finally got a bit of time back on it again today and yesterday. I fitted a set of OEM coils and leads because they were changed not long ago, no difference.

Got a complete set of carbs off a running bike - exactly the same problem! As soon as I give any throttle at all it dies. I've just checked the static timing again and it looks perfect so I'm going for another rectifier and then thinking ECU or loom. I am going to look at fuel pump again because I noticed the diaphragm of the original pump is leaking now too
 
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