18 replies to this topic

[Keith Grendahl]

1275 A, dual HS2's with M needles/blue springs.
Pertronix electronic ignition and non-ballast coil.
Non-vacuum advance 25D dizzy.

Has run very well with this set-up in the past but developed a stumble at cruise in 4th gear. Now it has a serious loss of power in 3rd but especially in 4th. Revs through 1st and 2nd up to 5,000 rpm, good acceleration. In third it won't usually go over 4,000 and in 4th it gets to 4,000 with wide open throttle but slowly loses rpm's down to about 3,500. At part throttle it has a slight stumble. Increasing the throttle to wide open results in acceleration up to 4,000 and again slowly loses rpm's. If I release the throttle and put it back to the floor, acceleration is good but it quickly starts to lose power. Repeatedly releasing and increasing throttle to wide open, I can get it up to about 4,500 rpm.

I used to be able to easily get up to 5,800 rpm in 4th. Cruising at 4,300 and 60 mph was the norm.

I've tried another 25D dizzy, cap and rotor, HT leads. Set tappets, repeatedly set timing and fuel mix, and replaced fuel filter. Doesn't seem to improve when making timing and mix adjustments. Running with air filters off for easy access to mix screws.

Haven't checked compression recently. About a year ago I replaced valve guides and seals but still uses about 1 quart oil/200 miles. Last compression was 145-150 across all 4.

It's good if I only want to dart around town through first, second, and some of third but I would like to get back to longer distance drives.

I'm considering putting points in to eliminate the electronic ignition. Or replacing the coil, but it seems to be a fuel issue.

All recommendations appreciated,
Keith

[CobraV8]

maybe check that you are not starved of fuel, fuel pump, filter and float level.

[jaydee]

Poor compression, maybe the rings are well worn, does it smoke under load and oil goes black after about 300 miles from oil change?

[carbon]

The carburettor piston springs should be red for this spec engine, you might be running slightly lean at top end, but you don't say what cam profile you have fitted which could make a difference.

Also may not be helped by the amount of ethanol being added to the petrol - which fuel are you using?

Also you mention using 25D without vac advance - what amount of centrifugal advance does this have and what static timing do you have it set to?

[bmcecosse]

Does sound like fuel starvation - but you should run a vacuum dizzy -and tiny twin HS2 are too small for a decent 1275. However I accept it was running 'ok' and now it's not! Basic valve gap setting followed by compression check is the first step.

[Keith Grendahl]

Set valve gaps yesterday so this evening just got it warmed up to check compression:

1 - 145
2 - 140
3 - 155
4 - 150

Good flow from electric fuel pump, lifted tops off bowls, didn't see any dirt/debris. Went for another drive. Excellent acceleration in first and second. Had it at 5500 rpm in second, got back up to 4,100 in third and felt it struggling, fourth was the same. Losing the rpm's down to 3,500 and struggling.

I sprayed starting fluid around all possible vacuum leaks. Slight decrease in revs on one throttle spindle.

carbon, carbs had red springs, I tried blue thinking the pistons were struggling to stay elevated. Using E85 93 octane, same as usual. Weight in the dizzy has 18 stamped on it. Static set to about 6 degrees. Recently I have been setting idle at about 2,000, advancing timing until revs decrease, back it off 150 rpm. Set mixture with lifting pins. When road testing I have advanced and retarded timing with no positive result. Cam is unknown, previous owner showed the car in the 70's so he made it appear to be an S with the carbs and dizzy, I'm assuming standard cam in the engine donated by an Austin America.

Tried the vacuum dizzy by connecting to carbs via Y tube but didn't notice any positive affects from it so went back to the non-vacuum because I knew the engine ran correctly with it in the past.

Clutch breather connected to carbs. No other breather installed. Vented oil filler cap. Revs decrease when cap removed, nearly dies. Same when hose removed from breather.

Without any recommendations my next step will be to remove carbs and clean/inspect though I rebuilt them only a year ago.

[bmcecosse]

The compressions are not great - but not hopeless either. That's a LOT of advance on the dizzy 36 +6 = 42 ???
Try a flow test with the pump - time it pumping two pints into a can.

Edited by bmcecosse, 01 October 2012 - 10:54 PM.

[Earwax]

Hi try running without fuel cap on - sometimes pressure build up works against pump. Mine was fine for short blasts but over time would slowly strangle - with more of a bootful required. ( Check caps and fuel breather hose if any)

[racingbob]

did you say the engine uses 1 litre of oil every 200 miles surely there is a big problem there

i thought mine was a lot at 500 miles for 1/2 pint and i got a slight leak

[Keith Grendahl]

2 pints approximately 30 seconds.

Fuel tank breather is clear and cap is vented.

Bores are worn, uses a lot of oil.

[Earwax]

Even though you changed the fuel filter there could still be water and or partly 'bonded' dirt - through the use of additives etc. Again on mine- symptoms very much like yours with the full on blap to the floor over and over would try and break free from fuel constriction but eventually strangled itself. Some electric pumps have a tiny filter on intake side?? - I emptied tank and cleaned as well as changed filter and 'bled fuel pump' Pain in the backside- hope you find it

[carbon]

E85 93 octane - how much ethanol does this brew contain?

Roy is right that with 18x2 = 36 degrees of centrifugal advance on the dizzy this is a lot. The Cooper S 23D dizzy had 30 degrees of centrifugal advance but also with a very heavy secondary spring. But with a lower compression motor you may actually need some more advance.

Could be running weak at high rpm. With first and second the carb piston damping effect may still be richening up the mixture under acceleration, this may not happen in third to same extent and certainly not in top.

Replacing the red springs will richen up the mixture, may help.

[Keith Grendahl]

Unfortunately I have not had the time to look at this problem the past few evenings. I will get back to work on it this weekend. Thank you, everyone, that have offered advice.

I need to clear up a few thing I misspoke. Not E85 petrol, that is 85% ethanol. I'm using a 10% blend. The dizzy is a 23d (40819).

I'll post again this weekend after putting the red springs back in the carbs, checking timing, mixture, etc.

[Pigeonto]

easy to overloook silly things thinking it's more sinister, have you tried it without the air filter ? I run anything up to 38 deg advance when cruising and it would take more (megajolt) but of course that pulls right back to much less under load and under load is where your problem is so it would want 15 - 20 there. I once heard of a blocked exhaust system which would definately give high rpm issues, and it could have got kinked,bit extreme but you have to consider everything.Does the car push around easy (brakes binding)

[Keith Grendahl]

Put red springs back in carbs. Went back to the beginning by setting static timing, balancing carbs, and setting mixture. When I "blipped" the throttle individually, the left carb caused a misfire. With linkage loosened, increased idle on each carb individually to see what happened. Right carb drove revs to 3400, left carb would drive revs to only 2600 and would decrease as idle screw was turned in. Adjusting mixture both rich and lean didn't produce any positive results. Covered the left carb inlet with my fingers and revs went right up 3800 and the misfire cleared. Checked for vacuum leaks again but couldn't find any.

I assume a vacuum leak. Carbs will come off tomorrow for an inspection.