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revving issues  Rating:  Rating
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 Posted: 04-18-2011 03:41 am
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jarnol123
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OK here's a new one.....I finally got the dellortos back on and I got the Jensen running nicely and idling at 1000 rpm in the garage...."Great" I thought....I'll take it for a spin...got it out on the road and i got the "bogging" problem...ie when I got it in first and pressed the accelerator it had no pickup...it ran but as i said didn't pick up...did some more adjusting of idle mixtures but to no effect....

 

Then something really strange happened...I turned on the ignition and started it and then it accelerated itself way up to 6000+ revs before I turned the ignition off...if I turned the ignition on just before it dies it'll rev back up again....btw I'm not touching the pedal at all.

 

I checked the cable and no problem...I just can't understand how, on it's own, it can rev up so far and so quickly on it's own??

 

Baffled??

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 Posted: 04-18-2011 04:40 pm
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jarnol123
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Also, I didn't mention this but I set the timing at 15 degrees BTDC.  In the garage it worked fine and it idled nice at 1000 rpm. as soon as I got it on the road there was bogging as soon as i started to/attempted to accelerate.

 

Maybe the timing has something to do with it trying to get to 6-7000 revs in 2 seconds??

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 Posted: 04-18-2011 09:56 pm
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subwoofer
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15 degrees is WAY too much at idle, you should be at 10 degrees at idle (without vacuum connected if you have Strombergs). Max advance should be checked, 28-30 degrees is correct.

Over advancing at idle causes all sorts of problems, so get it right before doing anything else. High idle when warm is typical if you have vacuum leak into you intake manifold, btw.

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Joachim

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 Posted: 04-18-2011 10:08 pm
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jarnol123
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Thanks Joachim...

Actually i think i might have a vacuum leak as well...I guess that would explain the high revving and maybe the bogging of the engine under acceleration?

I'll reset the idle timing to 10 and see what happens.

 

 

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 Posted: 04-18-2011 10:12 pm
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subwoofer
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Actually, I believe the bogging at take off is due to being over advanced at idle. This causes the butterflies to be far from the first progression hole at idle, giving you a bad lean spot just off idle. Too much advance does not mean higher power, rather less power and a greatly increased risk of destroying the engine.

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Joachim

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 Posted: 04-18-2011 10:29 pm
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jarnol123
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Ok so if I understand it the best thing to do here is check for vacuum leaks, set idle timing to 10 degrees btdc and go from there?

Also, out of interest, which spark plugs do you use?

 

Thanks

Jeff

 

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 Posted: 04-18-2011 10:38 pm
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subwoofer
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I run NGK plugs, BPR6ES. Nothing special, dirt cheap and works.

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Joachim

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 Posted: 04-21-2011 03:34 am
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jarnol123
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My dellortos have two "breather" ports under the K&N filter; looks like someone added that on. One goes to the engine and the other gors to where the oil filler cap is. I don't think this has anything to do with why it's over revving.

I took another look and I can't see where any leaks would be...I'm completely stumped as to what would cause the engine to over rev by itself....what would cause this to happen? would it just be timing? I don't think so?

Any ideas would be much appreciated.

 

 

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 Posted: 04-21-2011 02:15 pm
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Brett Gibson JH5 20497
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Sounds like someone plumbed the carbs to pull in the engine and valve cover oil vapors, usally most poeple would just tap the back of the air cleaner, if your not using it you should plug it off at the carb. Also Dell's most often do not use a distributor that has a working vacuum advance, if yours does I'd plug that off as well.

Thier are usally only two taps for vacuum on the engine one for the brake booster and the other for a distributor, (discused) if your booster is leaking that could be an issue as well, try plugging that off to see if it make any improvement.

Plus are the carbs tuned so that they are pulling the same amount of air flow equally.

Brett  

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 Posted: 04-21-2011 04:24 pm
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jarnol123
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thanks Brett - I haven't got to the point of tuning them yet...just want to get it running first. You know I spoke to a guy at Delta and he told me that the dellortos need to have a little "give" in them when reattached, ie not to overtighten them so as to squash the large o rings. Well mine is super tight so i bought some new o rings and will install this weekend. Is this right though regarding a little give? He said that you should be able to hold the carbs from ubnderneath and there should be a little play ?

Also Dell's most often do not use a distributor that has a working vacuum advance, if yours does I'd plug that off as well.

Wasn't sure what you meant about this bit...still learning...i have the standard Lucas distributor

 

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 Posted: 04-21-2011 07:00 pm
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John Finch
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If you don't already have this book you  need to get it.

"How to Build and Power Tune Weber & Delorto Carburettors by Des Hammill.

Delta might have it?

Best of Luck

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 Posted: 04-21-2011 07:26 pm
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subwoofer
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The book is essential for one purpose only; the proper procedure for stripdown and rebuild of the carbs.

Daily rant: I really don't understand why everyone thinks that book is so great, it just basically tells you to richen it up whenever you have an acceleration problem. No theory of operation, no references to (modern) equipment for diagnosis, no discussion of emulsion tubes and the effect of float level, no discussion on tuning for part-throttle economy while retaining top end power.

Back to the issue at hand: get yourself a CarbTune Pro or equivalent, that takes care of the balancing problem. Easy as pie to operate, and you have yourself perfectly balanced carbs in ten minutes the first time you try.

Second: there should be a 0.015" gap between the manifold surface and the O-ring holder, and the same gap between the O-ring holder and the carb. Do not tighten the mount bolts more than this. The carbs should have Thackery spring washers or rubber inserts under the mounting nuts.

Just unplugging the vacuum hose from the distributor is fine, just make sure you plug the other end. Revving all by itself is for now a bit of a mystery, but we should be able to chase down that after all else is properly set.

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Joachim

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 Posted: 04-21-2011 07:42 pm
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jarnol123
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Thanks joachim...and i agree about the hamill book btw...i need to get some of those thackary springs...delta didn't have any yesterday .

 

The carbtune you speak of is the Morgan carbtune i presume?

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 Posted: 04-21-2011 08:04 pm
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subwoofer
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That's the one. Worth every penny!

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Joachim

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 Posted: 04-21-2011 08:34 pm
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Brett Gibson JH5 20497
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I get the feeling your trying to drive a car that is not really ready to be driven, you should take a time out and go thru the ignition system making sure the plugs/points (if you have them) rotor/ wires / timing etc are all set before you muck about with the carbs. Once your set there then go thru the carbs, you cant really expect not to have problems if things are not correct.

As for "The Book" I think Des Hammill did a nice job with it and I especially like lots of pictures that go along with the explaination's, but thats me. There is also another book I have "The Dellorto superformance" Tech book by Bob Tomlinson which Joachim you might like better.

If you have a vacuum hose off your manifold going to the distributor plug both end's you dont need it with dell's. One more thing check those carbs for air balance if one is doing all the work you can toast your valves on the to lean carb side they need gas to lube and keep cool.

Brett

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 Posted: 04-21-2011 08:52 pm
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subwoofer
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After having all sorts of ignition problems last year, I agree with Brett that you should give the ignition system a good going over as well. Get a Pertronix Ignitor-II, a good 12V coil and get rid of the resistor (wire). Use the timing gun to verify you don't have sticking weights in the distributor, and that max and min advance is correct. Look for timing "jumps" on overrun for instance.

Syncing the carbs should reveal if you have vacuum leaks in the vacuum take-offs and connected systems, since you will have to open up the inner two bypass screws (a lot) to match the outer ones which will have lower vacuum if you have a leak.

And I agree; don't drive the car until you have the basics sorted!

As for carb information, the Innovate Motorsports forum has a lot of good information, especially the articles referred to in the "stickies".

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Joachim

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 Posted: 04-22-2011 05:12 am
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Jensen Healey
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As far as the Dess Hammill book goes, there is a wealth of information there that most folks will never bother to understand. There is a clear A to Z of how to tune any engine using basic tools. Thanks to this book I have sucessfully tuned my engine without outside help, once during the rebuild, and again when I installed the 107 cams.

It's not for beginners and certainly doesn't contain every bit of knowledge on the subject but contains enough information to get the job done.

I have tapped the power of the Dellortos with the help of this book and believe it is the best on the subject.

Kurt Housh

JH 13148

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 Posted: 04-22-2011 07:26 am
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subwoofer
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subwoofer wrote:
Second: there should be a 0.015" gap between the manifold surface and the O-ring holder, and the same gap between the O-ring holder and the carb.

Ooops! It suddenly struck me that I got the mm-to-inches conversion wrong! The correct measurement is 0.030" (0.75mm). Measure with a feeler blade.

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Joachim

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