Flat Spot

[LORENZO73]

Hi guys,
I am hoping that someone can help me with my car.
It has a 1800 super engine that has been recently rebuilt.
The car has a large flat spot until the car is doing more than 30mph (50kph)
The car is standard except that i have added twin HS4 SUs. The following has already been checked:

1. Cam timing is now exact following the rebuild.
2. The carbs have been balanced using a proper manometer to read & balance the vacum.
3. The mixture is spot on using a Gunson Colourtune.
4. The distributor has been checked and it has new points and condenser. The timing has been set up using a strobe light.

All advice welcome
Renzo

[balmy]

What camshaft is it running?

Twin carbs really need the TC cam and distributor to work properly.

[LORENZO73]

Thankyou David for your immediate replay.
She has 1.8 super camshaft and the distributor that she has is mini clubman according to the number on it.

I already did a rebuild of the engine i don't wish to open it again.
Renzo.

[locost_bryan]

The advance curve for an A-series Mini Clubman will be different to a B-series Marina. The ignition advance is likely too far off below 50kmh, hence it feels flat and unresponsive.

Ideally you need to get a Marina distributor, or change the weights/springs/vacuum advance unit to match the Marina, or get one of the tunable electronic distributors (with 16 different advance curves) so you can trial to find the one that gives the best performance.

[LORENZO73]

Thank you Brian for your replay.
Yes i think like you brian, and david.
So i want camshaft and distributer, gaskets and others thinks.
The think is that me i am not a mechanic, and now have to pay Labour for 2nd opening engine.

Many thanks for help
Renzo

[david painter]

I would not be pulling the engine apart for a flat spot. When the throttle is opened three things happen more air is drawn in via the open throttle plate the vacuum drops off the ignition is advanced via the mechanical advance in the distributor and the fuel mixture is enrichended via the carburettor in a SU this is dome by the rising piston in the carb.
In practical terms SU carb engines very rarely suffer flat spots other than caused by either no oil or too lighter grade oil in the dashpot. I have looked on Autodata tonight before leaving work and a mini has a much different advance cure than a 1800 Marina in either SC or TC vairiants.

If the car was in my workshop for me to diagnose I would approach the problem in the following order.
1 Check Compressions should be equal to within 5-10psi and be 175psi this confirms you have a reasonably sound engine. When Spark plugs are you set gap to 0.025 inch Tappets should be set to 0.013inch using rule of nine

2 Check inlet manifold depression (vacuum) downstream of the throttle plate at hot idle you should have a stable 18-20inhg a lower figure than this telly you either a blocked exhaust or late ignition ( retarded timing) a low pulsing vacuum (shaking needle on gauge) tells you that you have a valve sealing problem.
when all of the above carried out and any mechanical issues now ruled out, or corrected as required.

3 Next I would start with the ignition check the Dwell angle should be stable between 57 and 63 degrees

4 Check ignition timing set to 7 deg BTDC at 750-800rpm without vacuum

5 Now carry out mechanical advance check 2-8 deg @1200RPM 10-14deg @ 2200RPM 20-24deg @ 3600RPM 26-30deg @4800RPM
28-32deg@6000RPM this is done with vacuum disconnected and plugged. Proper TC figures are generally 2deg retarded all through the range. However in todays real world of todays unleaded fuel you will need about 4 degrees or retard to these figures to stop the engine pinking. The trick is to run as much advance as possible without pinking. When you have established and corrected you ignition curve as required. Check your Vacuum advance works suck on pipe or use a Mighty Vac with engine running and check ignition advances using strobe light.

6 Now you can move onto the carburation check jet heights are equal, initial setting is 12 flats down from level with the carb bridge HS Carb, or 2 turns on HIF carb Check pistons lift freely and drop down with a sharply with a clunk with the dash pot dampers removed. Top up dampers to top of piston rod with 20w50 oil and refit dampers.

7 Start engine allow to idle and balance air flow to equal on both carbs.

8 Set mixture again equal on both carbs if using a CO meter 3.00-4.50%, if using the jiggle pin method lifting pin should cause a rise and then a drop back in idle speed when mixture is correct.

9 Now road test car 3-5 miles car should now run correctly without pinking ( retard ignition slightly by ear) until pinking stops. When get back remove spark plugs they now with be a even grey brown colour as per photo in my brown coupe thread. Black indicates rich mixture, White lean(weak mixture) the plugs should be even in colour across all cylinders. If different tweak mixture on either carb as required and repeat process, until the even brown grey colour is achieved.

turning mixture nut down on HS carb richens mixture on HIF screwing in mixture screw richens mixture
Dave

[LORENZO73]

Many thanks Dave. You want to come in malta to settled this problem? Because here mechanics nothing now. Every day ara flights. I pay everything no problem. is better that me do that, before continue put money in the drain. For now i spent 600 euro for nothing. I don’t find a write person to errange this problem. Renzo

[locost_bryan]

I wouldn't worry about changing the camshaft, until you have got the correct distributor. If you can't find a good Marina distributor, then one from an MGB would work better than one from a Mini.

Fitting a TC camshaft would give you more power, and doesn't require stripping down the engine (can even be done without removing the engine, if you follow the instructions here. However, the TC also has a higher compression ratio, and you won't get that without fitting the correct pistons or skimming the cylinder head.

[david painter]

I can understand the frustration, if you were local I am confident this could be sorted fairly easy, A lot of this younger generation of Technicians as the like to be known have grown up in the plug in age, looking to the computer to solve the problem. Seems to me so much has been cut out of the training at the expense of learning how things work and if you don't know how it works you cant fix it. I get modern cars in for repair that have been Main Dealer that cant be fixed. Had a Ford Focus the other week with a bit of effort, it was a simple fix he even got his money back from the Dealer.
Dave

[LORENZO73]

So the flat spot is coming because i don!t have a good distributer, or something in the carbs? Not because she is 1.8 super but have two carbs and she didn’t have camshaft Tc ? Thanks Renzo

[david painter]

In my opinion it is more probable a tuning issue than a camshaft one. With std camshaft it wont deliver quite as much power as a MGB / TC cam. Providing your cam was correctly timed when you had engine to bits there is no reason for it to cause a flat spot. A Vacuum check of the inlet manifold depression will confirm this and the mechanical integrity of the engine
Dave

[MarinaCoupe]

However, the TC also has a higher compression ratio, and you won't get that without fitting the correct pistons or skimming the cylinder head.

Bryan - the Super and TC shared the same high compression pistons, it was the DeLuxe that had the low compression pistons.

[LORENZO73]

So I think, this distribuer is the problem, i try to find one for 1.8 super, then I hope that my mechanic make the things exactly that I read.

Renzo

[balmy]

Personally I’d start with a dizzy and go from there. Dave’s advice is spot on and the way to start.

Dave-surely swapping to twin carbs causes a decrease in intake speed at low revs which could cause the flat spot? What I don’t know is how the TC set up differs from a Super to minimise this?

I’m sure I remember a conversation with Chris on how a single carb 1.8 makes more low down torque than a TC with this being one of the reasons.

[LORENZO73]

Many thanks for your replay David,
I try first the easy way, that i change the distributer and all settings necessary, then if she continue give flat spot,
i change camshaft and distributer.
The problem is, that i want everythig with mechanic, so for money, and for days or weeks.

Renzo