Engine wont rev above 3000rpm

[Duane]

Hi Team .
So I spent Xmas holidays replacing rhe camshaft in my 3500. Kit was from Wins.I am certain I lined up the marks correctly when putting the new timing sprockets and chain on.
The engine now won't rev above 3000rpm at which point there us engine shake and backfiring from rhe exhaust.
The engine never ran right before hand so I don't have a benchmark.
Could it be a faulty distributor not advancing, or have I set the cam position wrong?
Car starts, idles and srived fine up to 3000rpm Timing set to 6dbtdc and dwell angle at 27 degrees. You can see the timing advance with a timing light at increased revs. Could the weights in the distributor be frozen?
Carbs are set right.

 

[jp928]

New lifters fitted with the cam? Correctly run for first 20 mins ~2000-2500 rpm? If you see timing advance as revs increase its likely the mechanical advance is working. Plugs leads correctly refitted? Incorrect plugs firing can give you backfires.

 

[Duane]

Yes new lifters and ran at 2209 rpm for first 20 mins.
I have quadruple checked the plug leads.
Surely if I have the timing chain set wrong it wouldn't run properly at any rom?

 

[keynsham1]

I recently changed a timing chain and gears on a TVR V8. I found the timing marks to be very odd as the actual alignment mark is not in line with the spokes of the large gear. I was trying to line it up with a molding mark on the wrong side of the gear wheel. I found that Rover P6, Rover SD1 and TVR manuals sometimes get this wrong, or are very misleading.

 

[Duane]

Yes. I did use the little dot as you have shown rather than the long line. That seemed to match how I removed the entire assembly relative to the keyways in the aprockets.

 

[colnerov]

What carbs has it got? Is there a restrictor in the manifold?

 

[ratwing]

I had something similar, it turned out to be one of the carb pistons was stuck and not moving.

 

[Duane]

What carbs has it got? Is there a restrictor in the manifold?

Standard SU HS6. I don't believe there is any restriction.

 

[Duane]

I had something similar, it turned out to be one of the carb pistons was stuck and not moving.

Interesting. I did have to replace one of the pistons as the needle screw was knackered. I had to carefully sand the perimeter down as it was binding in the housing initially. Will have another look at that.

 

[jp928]

Isnt there a test - piston falls under its own weight?

 

[colnerov]

Interesting. I did have to replace one of the pistons as the needle screw was knackered. I had to carefully sand the perimeter down as it was binding in the housing initially. Will have another look at that.

What did you sand down, the needle or the piston? One will change the fuel mixture the other will change the way it reacts or responds to the throttle. Did you centralise the jet? You have to remove the carb to see the restrictor, although you can see it looking through carb if you know what to look for.

 

[ghce]

Plug lead in the wrong cylinder?

 

[Duane]

What did you sand down, the needle or the piston? One will change the fuel mixture the other will change the way it reacts or responds to the throttle. Did you centralise the jet? You have to remove the carb to see the restrictor, although you can see it looking through carb if you know what to look for.

Very light sand around the piston circumference to stop it binding. It was very minor. I have tested today that the pistons rise and fall freely.

 

[ratwing]

I don't know if this makes any difference but maybe I should have mentioned that in my case the engine was well and truly worn and it was oily black stuff jamming the smaller diameter lower section in the carb body - on dismantling, the piston still moved freely in the dashpot.

 

[jp928]

Try to localize any misfires to a cylinder - that is, when its misbehaving, pull plug leads one at a time looking for one that doesnt change the problem. If you find one, try changing one thing at a time - lead or plug. I had something similar in an SD1 once - found the bad cylinder, changed the plug - no change; changed the lead, no change, changed the dizzy cap, no change - conclusion was problem was in the cylinder or the dizzy. Final fix - close the gap on the dizzy pick up down to .014" (minimum), because of weak magnet in dizzy .
I also once had a misfire in a NEW plug - it leaked air.

 

[Duane]

Thanks guys

 

[classicalgreen]

I had something similar, it turned out to be one of the carb pistons was stuck and not moving.

snap

 

[mikecoombs]

Given the engine runs ok up to 3000 I'd definitely be looking at the air intake side. first thing is to check you are getting full throttle with the pedal on the floor. Next is to check the movement of the pistons. You can see them rise and fall if you take the elbows off and should be able to lift them with your finger all the way up and let them drop (engine off so you don't go through the engine). Then remove piston and bell and look down the needle hole for the fuel level. You might need to pull on the choke (which lowers the jet) to see the level. Both carbs should have the same level and be a little bit below the top. That will tell you if fuel is getting into the carb. Does the engine refuse to rev past 3000 with no load? or only when driving?

To change the cam you need to remove the drive cam for the mechanical fuel pump. Did you put it back on and is the pump working correctly? Did you remove the heads, if so did you reseat the valves?

The pistons and bells are a matched set sanding them is a bad idea. Cleaning is best with carby cleaner spray. Likewise the needles are very specially designed and sanding them will certainly foul up the mixture.

 

[Duane]

Thanks guys. I have given the car to an old school mechanic as I was going round in circles. Seems like it could be the distributor that is messing up the timing and hence the vacuum.. Hopefully a relatively easy fix.

Mike - the carb piston was a second hand replacement from an SU specialist. It was just sticking slightly in the original bell, but some very light sanding of the edges with very fine emery cloth fixed it and had the piston falling at the correct rate.

I hoping for a successful fix this week.

 

[Duane]

Hey Team,

Still unable to find my problem although there seems to be intermittent improvement so I am coming back to fuel/air. I am certain the cam timing is correct having pulled the front back off again to check the marks line up.

Carbs are set right, but the mixture doesn't seem to be right. If I block the drivers side air intake the revs increase, if I do the same on the passenger side the engine dies.

If I have the mixture set 2 turns down on both carbs and I raise the piston by a couple of mm on the drivers side there is no change to engine revs. If I do the same on the passenger side the revs die. I have had to go 3.5 turns down to get the passenger side to behave like the drivers side. Is that typical that they can be so different? Jets and needles are brand new.

I have fuel getting to the jets, and can see fuel atomising when I look down each intake. Both pistons move freely full range.

I was also told by an old school guy that some setups had the fuel line return blocked off. Interesting because the line was blocked at the sender end when I first go the car and I thought it was just corroded over so I drilled it out. I've tried with the line clamped off but it doesn't seem to make any difference either way.

I have also set the timing down to TDC from 6 BTDC as per a couple of posts on here. Not sure what the consensus is on that, but the user handbook actually says TDC when running fuel lower than 100 octane, and I'm only on 95. Crappy New Zealand fuel supply.

Any clues? It is really doing my head in and I'm ready to set the thing on fire.