Will Not Rev Over 5000Rpm - Starting To Problem Solve

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Hello All

 

Got myself a TU3s engine recently and subjected it to a rebuild, with a few mild mods to the head / vale seats (nothing compared to what you chaps get up to ) and fitted a kent cams PT02 cam shaft.

 

On the rolling road today we got it running really nicely and it produced the power and torque of a standard TU3s on the rollers but when we revved it under no load we noticed it was just not revving above 5,000rpm? It was like a rev limiter was in place, it wasn't harsh it just stopped getting any higher.

 

We're not after more power here (although there may be a couple more horses to be liberated over 5K) we just want to make the car more driveable by allowing it to rev high enough to fall back into the sweet spot on the power/torque curve. As it wouldn't rev higher than 5K under no load,we've decided it's ignition related or possibly a broken valve spring, and that assumption is going to lead our next steps

 

I've search the forums for some help but this hasn't turned up any usefully similar posts. So I'm looking for some help and advice as I start to problem solve this.

 

Firstly - does anyone disagree that the engine shouldn't rev up around the 6,000rpm mark? Assuming not, here's my plan of action:

 

1. Replace the ignition amplifier
   • Does anyone know where to purchase the conductive gel that covers the surface, as I'd like to make sure that's not a problem too.
2. Try a new distributor
   • Does anyone have one for sale?
3. If 1 & 2 don't work, fit new valve springs

I'm particularly conscious that I'm entering uncharted waters and a lack of knowledge is quite dangerous. So If you're reading this and can offer help and advice, it would be very much appreciated.

 

Does it sound like I'm on the right track? Is there anything else I should be investigating/checking/replacing?

 

Have a great Xmas

 

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[jonXS 30 1 Cars]

Hey there buddy,

 

the ignition amp conductive gel, i just got some from maplins. Its the same as what they use on pc motherboards.

 

have you checked your distributors vacuum advance is working correctly? It could be just that? H&H could refurbish your dizzy.

 

and for the record i have on many occasions revved my tu3s to around 6k rpm, its not done any harm to it or me lol

 

hope that helps its no solution but it is a start possibly

 

jon

Edited December 14, 2013 by jonXS

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Funnily enough I'd bought some heat sink paste from maplins but you given me the confidence to use it.

 

Thanks for the steer on the vacuum advance...and H&H

 

All I want for Xmas is...6,000rpm

 

...so thanks v. much for taking time out to reply Jon

 

Cheers

 

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[trogboy 17]

Silly question but how are you measuring the RPM?

[allye 530 3 Cars]

That is a good point trogboy, those dash read outs are horribly unreliable. Pretty sure, in standard guise, these produce max power at something silly like 6250? Iirc. They don't actually have a rev limiter so should go beyond that.

[welshpug 1,661]

No limiter!?

Edited December 15, 2013 by welshpug

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yep, a good question Trogboy.

 

Then engine came with an unknown cam, which didn't suit the engine, rough and not very tractable...and it on the rolling road tester said the car felt like it wouldn't rev.

We put the new cam in and the engine is smoother and more tractable but the testers' opinion was that it was still being held back at the top end.

 

Off the rollers and under no load the needle hits 5000rpm and won't go any higher but the second choke kicked in at exactly 4000rpm on the needle. On the road this was the case too, with a change of gear at anything other than at or near it's limit risking falling below the second choke. I wouldn't have thought that would be the case it was actually revving to 5.5 or 6 (?)

 

I'm not convinced that's definitive evidence, but on the rollers and on the road it looks and feels to be the case.

 

Cheers

 

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[RossD 44]

6400rpm is peak power iirc for the standard TU3S.

 

Yep, no limiter on these. power drops off very quickly after 6400rpm, so revving over that isnt too much of a risk.

Anyway, both the TU5J2 and TU3J2 both have limiters at 7250rpm and share the same basic engine architecture as the TU3S, so its seems revs arent too much of a problem for these little engines!

 

Also the second choke doesnt "kick in" at a any certain revs. The carb linkage ensures that the second choke starts to open when the primary choke is 2/3rds open and by full throttle they are both open together. What you were experiencing was probably the effect the camshaft has on the engine.

 

Are you running a vernier type camshaft pulley on the aftermarket cam? If not, i would suggest this is your problem and more than likely your cam timing is a long way out.

Other things to check - Ignition timing, check both the base setting and that the timing swings to about 30 degreest BTDC at high revs. Also if you are on a rolling road, make use of the AFR probe and make sure you are not running either too rich or too weak. To change this, you will need to play with the jetting in the carb.

Also, make doubly sure you are actually getting full throttle when the pedal is floored - I've seen so many stupidly slack throttle cables meaning a floored accelerator is only actually about half throttle in reality!

Edited January 7, 2014 by RossD

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6400rpm is peak power iirc for the standard TU3S.

 

Yep, no limiter on these. power drops off very quickly after 6400rpm, so revving over that isnt too much of a risk.

Anyway, both the TU5J2 and TU3J2 both have limiters at 7250rpm and share the same basic engine architecture as the TU3S, so its seems revs arent too much of a problem for these little engines!

 

Also the second choke doesnt "kick in" at a any certain revs. The carb linkage ensures that the second choke starts to open when the primary choke is 2/3rds open and by full throttle they are both open together. What you were experiencing was probably the effect the camshaft has on the engine.

 

Are you running a vernier type camshaft pulley on the aftermarket cam? If not, i would suggest this is your problem and more than likely your cam timing is a long way out.

Other things to check - Ignition timing, check both the base setting and that the timing swings to about 30 degreest BTDC at high revs. Also if you are on a rolling road, make use of the AFR probe and make sure you are not running either too rich or too weak. To change this, you will need to play with the jetting in the carb.

Also, make doubly sure you are actually getting full throttle when the pedal is floored - I've seen so many stupidly slack throttle cables meaning a floored accelerator is only actually about half throttle in reality!

 

Thanks for sharing your experience Ross. I'll check all those things - especially the throttle cable! It's something I'd have never thought of, because it's so obvious.

When its been on the rolling road the AFR probe has been used and the ignition timing has been checked, although since posting I checked all the vacuum as per jonXS advice and found the carb base bolts weren't exactly tight so that might not have helped the vacuum advance.

At the moment the distributor is off as I'm looking to get it refurbished at H&H and I'm fitting a GTi twin fan and radiator set up - BTW re the top hose on this mod; do you know if two Samco 90 degree pieces (32mm diameter) and a hose joiner piece will do the job or is it another combination?

Thanks again

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[camgti 56]

Any more with this?

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Hello!

Busy time at work this last two months, but I got the twin fan set up installed and the engine filled with coolant last week.

Been an exploratory drive to help me sort out a few niggles - rally computer sensor, ballast for super oscars etc.

 

however, the important bit was the engine; and under load it's still not revving out. Driving has really highlighted that it's hitting the limit at 500rpm after the second choke opens. Otherwise it's running perfectly until that point.

 

I need to look at the throttle cable tension as RossD pointed out.

I've also read on a VW forum that a cambelt with the wrong number of teeth can restrict the engines ability to rev through the entire rev range.

As there as so many engine codes for engine which are all 1360 or 1.4 TU and/or mkII etc, I wonder if it may be set up with the wrong Cambelt - going to check this out too at the weekend. It's complicated by the fact that I know the age and type of the car 92 UK Rallye, but the engine is an alloy TU3s of unknown year. When I looked at some cambelts they sold two types of cambelt depending on whether the car was a MkII or not. This made me wonder if it might be the problem.

 

Any help with cambelt codes or advice much appreciated.

I'll keep you posted how I get on.

Cheers

Will

[RossD 44]

The cambelt type will depend if its an alloy block or a cast iron block.

If its a TU3S it will have an alloy block so will use the belt with the fewer teeth. I'd be surprised if you could get the longer cambelt for the iron block engine to tension properly to be honest.

[welshpug 1,661]

Uk rallye isnt a tu3s, its a 75bhp iron block.

[RossD 44]

Uk rallye isnt a tu3s, its a 75bhp iron block.

 

Indeed. But the OP has a TU3S.....

[welshpug 1,661]

oh, he said it was an UK rallye in the last pos't, didnt say the engine has been swapped

[RossD 44]

. It's complicated by the fact that I know the age and type of the car 92 UK Rallye, but the engine is an alloy TU3s of unknown year.

 

[welshpug 1,661]

blind in my coffee machine broken state

 

so, rebuilt the carb yet?

[trogboy 17]

Going back to the point I was going to make months ago, my TU3S used to fail to rev beyond 5750rpm on the rev counter, but in reality it was spinning much faster. ~7k at 5.5k indicated when I got it on a rolling road and we took the RPM from the pulses in an ignition lead.

 

If you are just doing it by the seat of your pants then you are probably being duped by the tacho readout.

 

To check the linkages on the carb just take the inlet off the top and look in. Manually turn the throttle cable cam and you can see the butterflies opening in each barrel. It's also good way of checking the accelerator pump jet circuit too as you can see the jets of fuel squirting out into the manifold below. Have you checked that you are getting full throttle when you plant your foot? This is easilty adjusted on the carb.

 

As Ross said, nothing about the second choke opening is related to RPM, it's al just about the position of your foot on the throttle. Plant your foot with the engine off and you still should fully open both butterflies.

 

Like Ross (again) if you are feeling a kick shortly before you run out of RPM it is probably the effect of it coming on cam. Seems very late in the delivery though. If there really is an issue it is probably related to the cam timing being out and I doubt that that is related to the belt. Check your vac advance works and the static timing on the dizzy.

 

I don't think you'd be able to properly tension the wrong belt, and if you could then it wouldn't make the timing wrong as the tooth spacing on the sprockets (and therfore belt) are the same for both belts, hence if the flywheel and cam are correctly located during tensioning the timing still works out, you just have more/less belt on the run around the tensioner.

 

If you were closer I'd come out for a passenger ride/drive and give you my opinion. I'm sure others would here too.

 

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Edited March 25, 2014 by trogboy