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Discussion Starter #101
Today's update - it's bang on level so I guess they did a good job of mounting it 'a la bike'

I have drained, flushed and checked all fuel lines. I've mounted, wired and plumbed the new pump and I fired it up briefly with the little fresh fuel I had left and it appears to be bogging down still. I quickly ran dry of fresh fuel so I'll pop another 10L in next time and make sure it's not just starvation
 

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Discussion Starter #102
OK fresh fuel in and it still seemed to break up at high RPM, airbox back on and sounds clear. I'll button a few bits up and take it for a spin soon, hopefully tomorrow
 

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Discussion Starter #104
Thanks!

Finally a little progress, had a couple of test runs, just up the road, it's running well up to about 6k rpm and then breaks up. The bottom half of the airbox is on, you may remember the cardboard top half? I've just made up a template in steal which I'm painting and then will foam seal it on and see if it's just leaning out at the top end due to the lack of restriction. Fingers crossed we're on our way to some success finally ...
 

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it's running well up to about 6k rpm and then breaks up.
What do you mean by '6k rpm and then breaks up', James?

Does it rev to 6k and then stall or bog down?

Just hesitate ...... or continuously run rough?

Can you rev through 6k to higher revs if you just ignore it and keep your foot to the floor (with due consideration for local speed limits & any speed-sensitive souls)?
 

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Discussion Starter #106
:)

It revs freely to 6k and then starts to bog down and run rough, hard to tell whether it's spark or fuel, feels like fuel. I didn't manage to push through it with light or heavy revs. Just finished the last coat of paint for the new cover so will make up some studs and fit that this week at some point and report back again afterwards, it's nice to have it half working again at least for now!
 

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Discussion Starter #107
Well, we finally have some progress boys and girls!

I cut a piece of steel, foam lined it and then fitted M5 studs to the airbox with some dome nuts to finish. Just took it out for a spin and it was revving freely up to about 6-7k RPM. I filled it with fuel and went for a small drive, absolutely perfect. The road cleared just before I got home so I gave it some throttle and it happily went up to 11k RPM without a stutter. I eased off and then it started bogging down but did clear after a few seconds. I gave it another little blast and it bogged down again afterwards but I ran out of time.

So I'm nearly there with a carb rebuild, non-contaminated fuel and an airbox not made out of cardboard and duct tape! I guess the new behaviour is due to mixture settings but will need a longer drive before I can assess that. I'll post up if it all goes horribly wrong but otherwise I think I'm almost back to running - Special thanks to Duncan and Ian and anyone who contributed in any way to this thread!
 

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Discussion Starter #108
Well, scratch that, just tried to take it out again and it's right back where I started. Dies with any throttle at all, had to push it back home. FFS
 

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Well, we finally have some progress boys and girls!

I cut a piece of steel, foam lined it and then fitted M5 studs to the airbox with some dome nuts to finish. Just took it out for a spin and it was revving freely up to about 6-7k RPM. I filled it with fuel and went for a small drive, absolutely perfect. The road cleared just before I got home so I gave it some throttle and it happily went up to 11k RPM without a stutter. I eased off and then it started bogging down but did clear after a few seconds. I gave it another little blast and it bogged down again afterwards but I ran out of time.

So I'm nearly there with a carb rebuild, non-contaminated fuel and an airbox not made out of cardboard and duct tape! I guess the new behaviour is due to mixture settings but will need a longer drive before I can assess that. I'll post up if it all goes horribly wrong but otherwise I think I'm almost back to running - Special thanks to Duncan and Ian and anyone who contributed in any way to this thread!
J, how does air now get into the air box? It almost looks totally sealed. In the original design there is a fairly large plenum with a reducer. I know it’s not the root problem, but certainly must be choking it off pretty good.

Well, scratch that, just tried to take it out again and it's right back where I started. Dies with any throttle at all, had to push it back home. FFS
I see in the second pic that you have an aftermarket regulator/rectifier. Any chance that white plug in it is fried on the inside, or inside the regulator end? Could be a power issue :idunno:

This makes no sense at all.

Any weird noises like a rod knocking or similar? I hate this because the entire set up is insanely simple. Gotta be an answer.
 

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Discussion Starter #110
There is a gap ~3x4 inches which is how it was when I bought the car and ran OK like that. You're right it will certanily have an effect but not the difference between yesterday (almost perfect) and today (can't do 10mph).

The rectifier is a second brand new one which I just wired in with the new plug, see post 90 (?!), the loom was indeed fried as you rightly predicted!

Way back before this started I was getting a knocking/tapping but I haven't managed to have it running to investigate. I did compression test and it was fine but if you have any other ideas I'm all ears

I was thinking that the pulse generator might be worth a shot?
 

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There is a gap ~3x4 inches which is how it was when I bought the car and ran OK like that. You're right it will certanily have an effect but not the difference between yesterday (almost perfect) and today (can't do 10mph).

The rectifier is a second brand new one which I just wired in with the new plug, see post 90 (?!), the loom was indeed fried as you rightly predicted!

Way back before this started I was getting a knocking/tapping but I haven't managed to have it running to investigate. I did compression test and it was fine but if you have any other ideas I'm all ears

I was thinking that the pulse generator might be worth a shot?
Ah, Ok. I try to remember everything :wink

Did this problem develop around the same time as the fried harness?

****Edit-Maybe the second hand Regulator is making you chase issues that aren’t there?

Any chance of getting another either new OEM one or an aftermarket mosfet one?

I’ve done both my 900’s with newer, larger reg/recs to try and stop any wire fires lol

Maybe it has something to do with it, maybe it doesn’t. I’m truly running out of ideas, save for a bad rod bearing that’s giving false compression readings.

This is truly a mystery I want to solve
 

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Discussion Starter #112
:) I understand, this saga has been going on for a while!!

The harness was a bit crispy, I noticed it when I bought the car but it didn't have an adverse affect (noticeably). I switched out the regulator early on and then switched it again during this investigation just in case and also did the loom

I'd very much like to solve it too :)

I guess I'll fire another compression test out at some point, I've got an ECU coming (clutching at straws a little I know) and I'll possibly switch out the pulse generator if none of that works
 

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A lot of time and work invested on this already, but the bloody thing is still not behaving consistently. Maybe its time to stand back and take a different approach - if it were mine I would make sure the carbs are all set identically, then take it to a (dynojet) rolling road and have a look at what is REALLY happening to the mixture -either with their tailpipe gas analysis or better, your own lambda sensor (air fuel ratio meter) fitted just after the 4-into-one junction. You could also get a readout of the ignition timing at the same time. You might save a lot of money this way, a fuelling setup will cost you around 300 pounds, a simple dyno run around 100.
 

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Discussion Starter #114
Thanks for the input, sadly with the issue it doesn't run well enough to even get to a dyno or rev at all under load. When I used the gas analyser on it, the mixture was swinging from rich to lean wildly.

I keep meaning to do the timing so will try and check that soon also
 

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Discussion Starter #115
New ECU made no difference at all, we're back to the state where it idles fine but dies under any throttle

Compression is all good still

I drained the carbs, ran some fuel through to fresh air in case any water was present again at the bottom, changed the fuel filter and no different

I had a look at the pulse generator and the connector was a little wet with oil, cleaned up and made sure disconnecting it kills it and it does.

Next I guess I'll run out of a fuel can again in case there's something in the plastic fuel tank which is wiping out the fuel within 24 hours that wasn't dispersed with the drain and flush.
 

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That's insane. Does it die the second you open the throttle? That would kinda point to something other than carbs, they should survive for a couple of seconds on what's in the float bowls. The only other thing that would respond so fast is if the spark was weak. At idle the pressure in the combustion chamber is lower, making it easier for a weak spark jump the gap. At higher pressures it might not make it. This might also indicate a dodgy low-tension fault in the ignition circuit. But I think you've been over that, right? This smells like the worst of all things, an intermittent electrical fault. Do you have a spare wiring loom?
 

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Hello James, I had missed the Fried Wiring Harness comments. I guess that even though it has been the case since you bought the car, it may have degraded? And may be affected by moisture. And, vibration. That is another can of worms entirely!
 

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I know you've been through the carbs a hundred times, but what does your choke mechanism and actuator look like. Any chance the choke plungers are sticking, making it cough and sputter under throttle? I do remember you had them out and 1 needle was broken off.
 

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I know you've been through the carb a hundred times, but what does your choke mechanism and actuator look like. Any chance the choke plungers are sticking, making it cough and sputter under throttle? I do remember you had them out and 1 needle was broken off.
I agree, I’m sure I mentioned this before, all the symptoms sound like a stuck choke
 

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It may be vapour lock! Just chucking it out there.
Give your fuel line routing the once over to make sure there are no hot spots.
 
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