I want to go slower but.......................

[mark030358]

Hi there,
Started running in my sprints engine this sunny weekend and have a slight problem.........I want to go slower!!!!

Ok the problem is that at over say 40 mph the car pulls great runs fine and is fabulous.

However, when I try and stay at say 30mph in top it and the best way to describe it is that the car gets "jerky". Seems like its missing. I have a feeling its the transition between idle and mains or maybe poor sync on the carbs.

Any one experienced this before? any thoughts on this most appreciated. MAny thanks to a MikeD for suspension advice by the way.

regards
Mark

[type26owner]

Hi Mark,
Assuming you even have Webers. Test for the lower rpm threshhold were the mainjets start to contribute fuel. To do this you put it in fourth gear and slow the car down until it's indicating 1500 rpms. At this point punch the throttle wide open and hold it there to do the observation. At first there will a small power surge from the accelerator pumps for less then a second. If it continues to run after that it means the mainjets are flowing. If it's not flowing fuel then the engine will shut off like you turned off the ignition switch. Find the minimum rpm where this happens. This test takes all of five minutes to complete. Report back here the results and I'll tell you what's the next step.

[mark030358]

Keith,
Thanks for the reply. One problem at the moment, my tacho is not working as I have fitted an electronic ignition (aldon). I will have to get a set of points, condenser etc this week and convert it back to get it to work. I was about to get my tacho converted, but its a 15 day turnaround.

The carbs by the way are dellortos, does the sameprocedure apply?

thanks
Mark

[type26owner]

Mark,
I've never had a Dellorto apart so I can't say for sure. Going on what others have said they are functionally almost the same.

Just do the test at 10, 15 & 20mph. I'll do the rpm conversion for you.

[mark030358]

Keith,
Took the car out and tried to do what you said. However the car is so uneven with it jerking and snatching at around 30 mph I was afraid of doing some damage as I am still running it in (100 miles only yet). However at just under 30 the car pulls away fine without hesitation.

I put a strobe on to check the timing and this seems ok at about 12ish degrees before. A couple of things of note are:-

1) the throttle is very sensitive to small changes in movement.
2) plugs are sooty after a 17 mile run.
3) jetting is as per lotus manual for what its worth.
4) as I said earlier car fitted with aldon ignitor system and lucas gold sports coil. non ballast type

Any ideas, this is spoiling a great car.

thanks
MArk

[mark030358]

Keith,
forgot to add it appears to get worse as the car warms up but could not guarantee this "fact" maybe my imagination.

cheers
Mark

[type26owner]

Okay, just a wild guess but it sounds like you're running too fat (rich). The feeling is normally described as the engine is surging. Also with that much fuel it should be exhausting black smoke but you may not be able to see it. Someone following you could though. If this is the case then downsize the mainjets by at least two sizes and test drive it. Keep going smaller until the engine starts to misfire. Backup away from the lean misfire by two jetting sizes. This technique is extremely crude and not a very good way to tune but it will make the car drivable. There is no way to adjust the mixture very well other than getting a CO or Air/Fuel meter to do so. The AFM is preferable by at least a factor of ten over a CO meter.

The airflow balance between the carbies can be quickly tested by just slowly running up the idle speed to 3000 rpms while stationary. If the engine starts to shake around violently then that could be your problem also.

[Hamish Coutts]

Mark,

A long shot - are the carbs mounted properly? By that I mean if they are loose (too big a gap on the thackrays) they may wobble a bit on their mountings and let in air past the O rings.
This can amplify as the engine moves on its mountings and shakes the carbs making them let more air in and so on.
Had this problem once on a lotus cortina more years ago than I care to remember! My carbs were too slack.



Hamish.

[mark030358]

Keith,
Your right about the car being rich. Took it for a 70 mile jaunt last night and kept the rpms to approx 3000 (and the speed to 50). Got home and above the exhaust pipe is black as are the plugs. However, at 3000 rpms and under very little load I would have expected to still be on the idle jet side of things.

As an aside my tacho started to work. Connected the coil supply from the fuse box and away she went. (thanks for the help in the other posts!)

Anyway one thing I noticed with my now working tach is that the idle rpms are well up when hot; so I'm wondering if my carb sync is poor and contributing to my woes.

There is also a faint misfire when the throttle is being feathered at lower rpms.

cheers
Mark

[type26owner]

Mark,
Actually you're fully onto the mains by 2000 rpms if your chokes are not sized too big. You can check this easily by doing this simple test. In fourth slow it down until you're at say 2000 and romp fully open the throttle. The first fraction of a second the engine will lunge slightly due to the squirt from the accelerator pumps. After that if the engine does not act like you switched off the ignition then the fuel is being provided exclusively from the mainjets. Why? Because the at WOT the induction manifold vacuum pressure goes up almost to atmospheric. No mixture will come out of the idle or progressive holes circuit under that condition, period. That test is a simple but very effective way to test for the largest choke you can use without creating a flatspot in the middle of the rpm range.

Guessing you've not got the idle circuit rpm tuned incorrectly relative to the first progressive hole. This is almost impossible to do without the proper tool which is an Air/Fuel Meter or second choice a CO Meter. The mixture strength is important and the normal way you're told to do it by seeking out 'the best lean idle' is a farce. Anyway when you can dial in the correct mixture you then need to find the required rpm so when the throttle is opened the slightest amount more it starts to suck fuel out of the first hole. Set the rpm too low and you have a stumble. Set it too high and you have a lazy idle were it's very slow to return backdown to the desired idling rpm or worst yet it hungs up and will not comeback down at all particularly when warm.

The airflow sync must be done at two different rpms and ways. The balance between the carbies is only really relevant at partial throttle at midrange of the rpms say 2500-3000. Never use up that center connecting linkage adjustment at the idle speed. All the folks (particularly the Weber vendor so-called experts) that recommend that should be fitted with a glass belly button. The idling airflow balance must be done on the older carbies by drilling a hole through three of the throttle plates. The newer carbies have an air bleed screw provided so the fuss and bother factor is nearly zero on those ones.

The slightest of misfires with a trailing throttle is normal while the engine is slowing down. With a full lift the manifold goes to it's highest vacuum and sucks really hard on the idle circuit which will being contributing some fuel. If you've got this right though there will never be any popping or backfiring, nada. At a steady throttle and rpm setting it should always fire cleanly. Chances are you probably don't have the progressive holes jetted correctly either. To my knowledge I'm the first one to figure out how to do this by removing the emulsion tubes and driving the car. There's help on the way when I shoot the video all will be revealed and made very simple.

Thanks for the feedback! Glad I could help!

[mark030358]

Keith,
Thanks for your last post.

However, slightly misled you with regard plug colour. When I got back home last night I looked at the exhaust and the back of the car and saw black soot. I then pulled the plug on No4 cylinder and it was black, SO assumed thet all were. However, 1& 2 plugs are light tan coloured, 3 is a dark tan to brown while 4 as I said is black. The idle mixture screw is only half a turn from the fully closed position so I don't think I can lean it out any more on this. So does this suggest I have the sync way off?

Finally how do you jet the progression holes other than by moving the butterfly position?

cheers
Mark

[type26owner]

Mark,
Check the resistance of the #4 spark plug center electrode. If it's a resistor type it might have developed very high resistance. Resistor plugs should be 5k to 10k ohms. Next check that plug wire. Easiest thing is to just replace the wire and the plug and see if the miss goes away. If it still misses then the cap might have a carbon track from #4 post to earth. You'll have to replace the cap, cleaning is not possible. Always make sure the electrics are perfect before suspecting anything is wrong with the carbies!

The progressive holes' mixture is controlled by the idle jets. Good choice at naming those jets if your intent was to confuse folks.

[mark030358]

I really still do want to go slower. Anyway progress to date and this includes stuff off other posts relating to my tacho problems.

Well, took out the ignitor and put in good old fashioned points and all other new bits and hey presto my tacho works fine. Just a small calibration error! Its been converted to voltage pulse type. As an aside sent aldon in the UK my oscilloscope traces for them to look at.

Carrying on........still wont run below 33 ish mph without kangarooing around. Have noticed however, its gets worse as the more miles (perhaps hotter the engine gets) I do.

Any thoughts anyone?

cheers
Mark

[mark030358]

Ahhhhh........... At last I can go slower.

What a pain this has been.

Anyway, replaced the diff due to rumbles, squeaks etc. Now the car will drive at 30 mph and slower without lurching or kangarooing around. Took it for its first blast since fitting the diff and what a DIFFerence (:-)) totally fabulous.

thanks to all for your help

Mark