3.2 K revs affecting SUs Vibrations?

[MichaelH]

Hi Everybody,

Driving along the motorway, I found that about 3.2K  revs that the engine appears to miss and loose power., it doesn't happen at 3.

Is there a vibration at that level that affects the SUs? and is there a solution apart from not doing 3.2 which normally is about the maximum that one can use within the legal limit

\i have looked at plugs, electrics checked plug leads for resistance.I am running an old 123 dizzy which I reset. Mechanical fuel pump- recently rebuilt.

Appreciate you opinions

Regards

Michael H

[RobH]

Assuming you don't want to exceed the limit - what happens if you drop a gear, or come out of OD, and cruise at higher revs ?  Have you tried a different ignition coil ? 

 

Edited January 28, 2023 by RobH

[rcreweread]

If it does it at all revs above 3.2k rpm , I would suspect fuel issues- maybe float levels set too low

You also mention you have a rebuilt fuel pump - maybe this is the issue - did you get it rebuilt or did you do it yourself ? Is it a repro or an original AC Delco one - there are a lot of poor parts out there

cheers Rich 

[Motorsport Mickey]

52 minutes ago, MichaelH said:

Hi Everybody,

Driving along the motorway, I found that about 3.2K  revs that the engine appears to miss and loose power., it doesn't happen at 3.

Is there a vibration at that level that affects the SUs? and is there a solution apart from not doing 3.2 which normally is about the maximum that one can use within the legal limit

\i have looked at plugs, electrics checked plug leads for resistance.I am running an old 123 dizzy which I reset. Mechanical fuel pump- recently rebuilt.

Appreciate you opinions

Regards

Michael H

Yes Michael, that’s in the area for vibration on the SUs. Try slackening the securing nuts a little to see if you can get rid of or move the vibration slightly to prove it. If so Thackery washers or rubber o rings around the studs can help 

Mick Richards

[Andy Moltu]

Apparently this also hits the 4 pots in the mid to high 5000s too on SUs. Rapidly accelerating through the gears can get past this so worse in higher gears. 

The TR7s had flexible mounts for the carbs to manage this. 

Presumably this is why you notice this in high gear /od on the motorway.

 

[stuart]

What carb insulators do you have fitted?

Stuart.

[MichaelH]

hi Stuart

manifolds go straight om the head with high temp silicone, carbs have the rather dense grey pieces between manifold and SUs.

All bolted up quite tight

I have weber carbs on another car which have rubber  "inserts" between manifold and carb- is there an SU equivalent ?

Michael H

Michael

Edited January 29, 2023 by MichaelH

[MichaelH]

Hi Rich

Original AC Delco with new diaphragm and new rubber seal underneath

The car accelerates well in lower gears thru to 5+ if pushed

The problems was that I did about 30 miles down the motorway this developed at 3.2 revs - we stopped it was the OK and ran normally again and on the way back we just ran down the motorway at 3 without any problems

I haven't changed the coil which is another possible

Michael H

[stuart]

1 hour ago, MichaelH said:

hi Stuart

manifolds go straight om the head with high temp silicone, carbs have the rather dense grey pieces between manifold and SUs.

All bolted up quite tight

I have weber carbs on another car which have rubber  "inserts" between manifold and carb- is there an SU equivalent ?

Michael H

Michael

TBH then I think I would be inclined to strip the lot off and start again, clean that silicon off properly and get a good grey material gasket, fit with a wipe of copperslip, then use the combination of the thin pink carb gasket the fibre type insulator then another pink gasket to carb. Ive used that setup many times and not had a problem. The Mesab isolators for Webers are only two bolt fittings. Has this just developed or been there all along, FWIW do you have the "Lump" fitted to the gearbox?

Stuart.

Edited January 29, 2023 by stuart

[MichaelH]

Hi Stewart Thanks for your thoughts: no lump fitted to the gearbox and overdrive. Problems developed recently when changing from webers back to SUs to get better mpg

Michael H

[Bfg]

Katie, my own TR4A suffers engine vibration at these same revs (..65 mph in o/d ) so I'm very interested to read that it may be attributed to the mounting / sealing of the SU carbs.  My car is fitted with the gearbox vibration-damping 'lump'.  I can drive through the vibration patch and things go smooth again at 70mph, but that patch does seem rather flat, as if the engine comes on cam thereafter.

Pete

[TR4Tony VC]

Mike

This sounds like you could have a moving obstruction in the fuel line, probably affecting the back carb. I had very similar trouble with my 4a some years ago and after I blew the fuel lines out (including the carb joining pipe) a small piece of pain flake flew out …. Problem solved.

in my case the back carb was affected, which is why the problem kicked in at 3000 plus rpm as the engine and fuel load down the pipe crept up. 

If you’ve lined the fuel tank this also tends to happen as the lining paint starts to lift.

worth a try.

 

Regards

Tony 

[Lebro]

I had the same, but it was almost a complete blockage. It was after the link pie had been off for a while, hanging in the garage while I was working on the engine. Took a while to find, but compressed air sent something flying across the garage, never found it, but suspected a spider !

Bob

[Andy Moltu]

If it were an obstruction, why would it be possible to drive through it and not continue at higher speed/fuel demand?

[MichaelH]

Thanks everybody:

Advice: to go back to proper material manifold gasket and pink gaskets: add the under gearbox lump and blow out the fuel lines, My tank has never had anything added to it in the way of lining.

Time to do some homework

Regards

Michael H

[Waldi]

2 hours ago, Andy Moltu said:

If it were an obstruction, why would it be possible to drive through it and not continue at higher speed/fuel demand?

Good thought Andy, I agree.

Waldi

[MichaelH]

Got a result and it's a long story, findings were that the car would start to miss fire  if driven at higher revs. If I started to run the car in anger(?) or just try harder as soon as the car had done a couple of miles it would struggle onto three cylinders.

I had previously had a petrol in the oil situation and sourced a lower AC Delco seal to stop it happening again and rebuilt the Fuel pump with new diaphragm, new seal and new one way valves,. The lower seal is kept in place with a retaining metal cap.

** I thought** "inadequate fuel delivery" - ie as soon as the carb bowls were empty the fuel was not delivering enough-> 3 cylinders and no power.

You have guessed by now : on re-examination of the pump the retaining cap had drifted loose and was halving the diaphragm  movement : so halved the petrol per pump: OK at slow but not enough at higher revs.

The solution was to use a screwdriver between the spring under the diaphragm to push down the seal cap( hopefully with slight distortion so that it stays down), then build the pump again and replace.

Thus far the engine will rev better and continue without going onto three.

Clever to find a replacement seal but not clever enough to to set it in place well enough

Everyday is a school day, mind you I have in the interim replaced all my petrol hoses, blown through all the piles, replaced by turn distributor , coil, plugs and checked the carburetor floats

I am just glad i got there in the end!

Michael H

[Lebro]

Well done for finding it

Bob

[stuart]

Weve all had that strange fault that after replacing half the car turns out to be something very simple!

Stuart.

[Motorsport Mickey]

Yep...almost like when you are searching for something...why is it always the last place you look. ?

My system for finding faults now is to retrospectively go back in chronological order and check ALL the "improvements" and changes I've done since the car was last running fine.

"Fitted Fuel pump with new diaphragm, new seal and new one way valves,. The lower seal is kept in place with a retaining metal cap."

Almost a 100% chance it's something that happened when you last worked on it.

Mick Richards  

[MichaelH]

Hi Back again

Thought I was excited that I had solved my problem:

No back to driving out of our village catch the traffic lights to turn right and accelerate - 300 yards up the slight incline and we are back onto 3 cylinders and loosing power

By the time I have got over the hill and slowed to turn into a thirty zone the engine catches up with itself and cruising thirty in top/OD we get back onto four smooth cylinders

Not dispirited (!) I take off the AC Delco mechanical pump and bolt in an old electric fuel pump I have: 4 psi and made in Japan and connect it up with petrol pipe.

I drive out  gingerly at first then with more aggression and so far it accelerates satisfactorily. Local dual carriage way running with the commuters getting home, several goes seem alright.

Will somebody explain to me why a mechanical pump won't deliver enough petrol to accelerate a cooking engine, 87mm piper yellow, on SUs E5 petrol

I have had the car since 78, raced it in the 80s, road rallied it in the 00s, even did a sort of track day at Curborough sprint course with my daughters. Have used SUs or a pair of 45 DCOE webers and until now always used the mechanical pump?

I have had the pump apart several times with new valves, diaphragm and new lower seal

Any more ideas?

Michael H

[RobH]

Is the drilling from the strainer bowl to the inlet valve clear of obstruction to allow sufficient flow into the pump?   

[MichaelH]

Yes checked: new metal pipe bolted in  clear hole into the bowl, brass mesh clean and complete.

Just gone back took the pump apart again and ran a brass welding wire through. Small hole  between the bowl and the first valve but not obstructed.

This may be the problem I will look through the other  pumps I have to see if the equivalent hole is larger in them

Thank you RobH

Edited June 27, 2023 by MichaelH

[Lebro]

Is the diaphragm moving enough, altering the number of gaskets between block & pump alters the amount of movement.

Bob

[MichaelH]

Just one paper gasket