Engine will not rev above 1500 rpm

[richardl46]

Apologies for gate crashing this excellent forum once more. My car is a 1972 Lotus Seven S4 with twin-cam and Weber DCOE 40's. The car has not been used since 1991 so after 2 years of rebuilding I was delighted to get it through an MOT test yesterday.

My problem (well the most serious one) is the engine refusing to rev above 1500 when on the road. Fortunately the car is light and very tractable below 1500, so changing up just before it dies enabled me to make the short journey to the testing station. Even when stood the engine is reluctant to rev above 3000 and opening the throttle fully is like switching off the ignition.

The only item not completely stripped before rebuilding was the distributor advance mechanism, although I did fit electronic ignition, new cap and leads.

Before I start working through possible causes, could someone point me towards anything that may be causing this "governor/rev limiter" affect? More details of the engine spec' and history provided if required.

Thanks, Richard

[Chancer]

engine only running on the idle and progression circuits, not the main jet circuit although for it to afflict both carbs at the same time is doubtfull, I had the same thing when a aux vent fixing screw (later webers) came loose allowing the venturi to turn and cut off the fuel supply, as it was only one carb the starvation occured at higher revs.

Check for the correct location and orientation of the aux vents, shine a torch down while revving to see if any fuel comes out.

[Concrete-crusher]

Your lucky my elan does not like to go below 1500 rpm ??

Seriously it could be a number of things one which jumps to mind is the ignition coil failing under load

Good luck Steve

[richardl46]

Appreciate for the suggestions guys. I've a feeling it's going to be something simple and obvious, but was hoping to get a short list of items to concentrate on.

At least now the car has an MOT I can test it after making adjustments.

Thanks, Richard

[promotor]

Check the fuel pump diaphragm and valves - could be not pumping due to either of those. A lack of fuel would cause your engine to die.
If it does it while the car is still cold then I would dismiss (for now) a faulty coil. Good quality coils rarely fail unless dropped, or cooked!

There's only ever two issues - fuel or spark!

[nmauduit]

engine only running on the idle and progression circuits, not the main jet circuit although for it to afflict both carbs at the same time is doubtfull, I had the same thing when a aux vent fixing screw (later webers) came loose allowing the venturi to turn and cut off the fuel supply, as it was only one carb the starvation occured at higher revs.

Check for the correct location and orientation of the aux vents, shine a torch down while revving to see if any fuel comes out.

My bet is like Chancer's on this one : carbs issue related to the main circuit(s).

If it was the fuel pump restricting fuel, once the bowls full the car would run fine at all revs for 5-10 seconds. If it was the advance mechanism stuck at 12? it still could rev higher than 1500. The clear symptom to me is the throttle opening, which leads to transition from sucking off the idle/progression holes to getting nothing from the main circuit.

Have the carbs been cleaned / dismantled recently (would air correctors have been forgotten or something) ?

[Elanman68]

Worth checking it's not over fuelling as it transitions to the main circuit as well. Pull the plugs and see if they're wet fouling. I had this on my Elan and it was the chokes not making a tight seal in the carb body allowing fuel to leak into the inlet, therefore being over rich and wet fouling. Fuel levels in the carbs are also critical as too high will cause spill over.

[richardl46]

Carbs were dismantled as found (in box of engine bits) and rebuilt 2 years ago, with new parts from overhaul kits, plus sealed shaft bearings. They are DCOE 40-18 which I gather could be quite old. The bodies were thoroughly cleaned and blown out, float heights set and all looked ok to my inexperienced eye.

I am having a little trouble sealing the misab joints and suspect both carbs and manifold faces may be slightly distorted. I'll use a little silicone when refitting.

Time to get the carbs off, compare all parts to exploded diagrams, and check all drillings from float chambers to jets. The general consensus of no fuel getting to main jets will be the main focus for now, as I am sure the float chambers are filling normally.

Thanks again for your input.

[mbell]

I wouldn't remove the carbs, remove the top cover and main stacks. Check the stacks and check the fuel level down the main stack hole. You should be able to run the engine on the idle circuit to run the fuel pump with no issues.

If you don't find any issue then maybe remove them.

[Quart Meg Miles]

It is worth looking at the ignition timing to see if it moves at all as you raise the revs, or moves drastically. I would hate to point a finger at the electronic ignition but it is an unnecessary complication, and any problem there might show up while you're looking at the timing.

I would have thought the engine would have revved above 3000 rpm without the main jets contributing much.

[Chancer]

I made a mistake on my first ever DCOE rebuild, old carbs like yours, symptoms dofferent though, it would rev in the garage and drive OK for 20-30 seconds before getting fuel starvation.

The old carbs had crenellated washers Under the jet covers, I now know they were to vent the float chamber, I used the sealing washers that came in the rebuild kit and not the vent covers, if your carbs dont have a drilling about 1/2" diameter going through to the airbox or if the airbox/filter does not have a corresponding drilling the float chambers wont vent on these early carbs without the crenellated spacer ring.

Part of the problem of diagnosing these postings is that unless the OP is an old git who drove old bangers in period they will not know the difference between a misfire/ignition cut-out and fuel starvation, if you have that ability it removes 50% of the potential faults.

And on that note, another suspect could be the rev limiter rotor arm.

TBH with the quality of modern repro parts I am surprised that any vehicle would start after a rebuild, they probably have multiple chocolate teapot components conspiring against it.

[Chancer]

I would have thought the engine would have revved above 3000 rpm without the main jets contributing much.

You are right, off load, it will rev up very well on the transition circuit, the car I had the problem on would drive up to 4500 RPM if the throttle was ony used a few degrees, any more or any higher RPM and it cut dead, that vehicle a Zetec 7 had a very poorly selected carb set up, it should have been fully on the main circuit well before 4.5K rpm equally the OP's carbs if its the same problem should be at a higher RPM.

The OP should confirm if the engine will rev any higher when not Under load.

[nmauduit]

I would have thought the engine would have revved above 3000 rpm without the main jets contributing much.

You are right, off load, it will rev up very well on the transition circuit, the car I had the problem on would drive up to 4500 RPM if the throttle was ony used a few degrees, any more or any higher RPM and it cut dead, that vehicle a Zetec 7 had a very poorly selected carb set up, it should have been fully on the main circuit well before 4.5K rpm equally the OP's carbs if its the same problem should be at a higher RPM.

The OP should confirm if the engine will rev any higher when not Under load.

yes, for fuel to be sucked from the transition holes you need strong laminar air flow there, which happens only when the butterflies are opened very slightly (it's not a question of load really but rather of throttle opening) - then that would confirm what works (idle circuit) and what not (main circuit), not necessarily why (fuel main circuit could be plugged etc)...

[richardl46]

Plenty of excellent ideas for me to follow through this weekend.

Will get back to you all with progress, as and when it happens.

Thanks, Richard

[richardl46]

Lotus Seven S4 T/C

Time to admit a senior moment!
Working through your list of suggestions I removed the Pipercross air filter and realised the back plate was covering the float chamber vent holes. Thanks you Chancer for pointing out the purpose of these holes.

My excuse is that I was fixated with making sure the bonnet did not foul the filter. For some reason Pipercross make the filter asymmetrical and my bright idea was to place the offset aft, so providing maximum clearance. The attached photo shows the filter in it's new position which just touches the inside of the bonnet.

Road test followed and problem solved; in fact the performance is positively vicious!

I am told by an owner who sprinted the car in the 1980's that the head was modified to fast road by Vulcan Engineering. The block was line bored, balanced and lightened by Baldyne Engineering. Powerplus pistons and Piper magnum 285 cams completed the job. The bores are +20 and crank -10. I rebuilt with new bearings and rings as a local machine shop found negligible wear and just honed the block. Is it possible I may have 130bhp on a good day?

My thanks to you all for the excellent response. I can now get on with the various minor issues, knowing the car is now a runner!
Richard