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MG MGB Technical - Somebody please explain this,

I drive a 1969 mgb roadster daily. I often take trips of 120 miles up to columbia SC to work on my MG V8 conversion. To ease these trips I installed a cruise control system on the car. It worked great for a while but then started having trouble(wouldn't stay on). So I installed a vaccume resovoir off of the camaro I striped for my V8 Conversion hoping to remedy the problem. Well, it didn't solve the problem, but what it did do was

1) Make the engine pull slightly harder
2) Make the engine spin slightly faster

I can definatly hear the engine spinning faster under hard acceleration with the resovoir than without. The car idles fine, cruises fine, and is slightly difficult to start initialy(until a vaccume is built up int the tank). My question is,what the heck is making the engine spin faster with the vaccume resovior, shouldn't it spin up slower? Beats the heck out of me and my father. Any ideas?http://www.hononwah.freeweb-hosting.com/Index.htm
Robert

Robert. Perhaps the reason no one has answered this is that we do not understand what you are asking. It would seem that you have a 1969 MGB with a four cylinder engine and you have added some form of aftermarket cruise control to it. While the cruise control initially worked properly, it began to malfunction. --Did you determine the exact cause of the malfunction? Did you contact the manufacturer of the unit for their input on what might be the root cause of the problem?

You then added some form of vacuum reservoir into the system. --Why? What was the desired outcome of this addition? Were was it added and how does it function in the system?

You seem to feel that the addition of the vacuum reservoir has, somehow, changed the engine characteristics. You say the addition makes "the engine pull slightly harder" and makes "the engine spin slightly faster". What does this mean? Does the engine pulling harder imply an increase in performance, as in "a hard pulling engine"? Does the "engine spinning faster under hard acceleration" imply that is is reving more freely? You state this is something you can hear. What about the tach? It almost seems that what you are indicating is that the addition of the vacuum reservoir did not cure your cruise control problem, probably because you never found the root cause of the problem, but had the beneficial side effect of improved engine performance.

What are you trying to say here? Les
Les Bengtson

Also, it might be useful if we knew which aftermarket unit you had installed. Many times things happen to our cars which are only coincidental to something we may have done to the car and are not truly the reason for a change in the car's behavior.

The vacuum cannister is used, as I understand it, to simply supply a resevior of vacuum to the vacuum operated "motor" that pulls the cable that moves and HOLDS the throttle linkage at chosen point. It buffers the minor variations in vacuum from the engine and makes the cruise control operate much less erratically and makes it easier to maintain a steady speed.

I think you need to look at your tach or hook up a separate Tach/Dwell meter as Les states to be able to get a good handle on whether or not your engine is actually spinning faster or if what you hear is possibly something else. If, indeed, it is something else, then the use of the cannister may only be coincidental to what you are observing. Or it may come about because of something like a leaky cannister and the noises something like that might make????
Bob Muenchausen

The unit came off a camaro. He should take it to a Mr. Goodwrench.
glg gimbut

You are right- my question could have been stated much more simply:
1)I drive a 69 Roadster
2)I striped a camaro(1 part is later used on MG)
3)I installed a cruise control on the MG for long trips
4)The cruise was from sears and was very old at the time, but unused
5)The cruise has trouble staying engaged
6)I install a vaccume resovior from a Camaro to attempt to remedy the problem
7)The problem is not solved
8)while out driving I stand on the throttle hard and notice that the car is traveling at a higher rate of speed and the engine pitch has changed when the engine runs out of RPMs
9)My tachometer will not read any more than 4500 RPMs
10)I disconnect the resovior and close the vaccume port on the intake manifold.
11)I go for another drive
12)When the accelerator is floored and the engine has run out of RPMs, the car is traveling slower, the engine sounds like it is turning over more slowly.

I know this is a long post- I hate long posts. The answer is not necessary to continue The existance of the car. "I have no clue" is an appropriate answer.
I was simply courious
Thank you to those who have responded
Robert

Hi Robert

Interesting....

I wonder if the reservoir is effectively smoothing out the pressure pulses in the inlet manifold ?..

Is this what the 'plenum chamber' is about in modern cars ?...

Do you know if there is a non-return valve in the reservoir ?... if there is, and it is working, I can't see why the reservoir has any effect.

Don
Don

There is no check valve in the reservoir. All that I can figure is that the vaccume it stores helps to lift the dashpot when the throttle is opened. I can't think of anything other than this. It did supprise me quite a bit- but even puzzeled me more.
Robert

Can it be that more air is getting into the intake with the reservoir installed? Wouldn't this cause a lean effect which would seem to present the symptoms described? If so, when the "vacuum port" is closed off, the mixture would go back to normal which may seem "slower".

One more thing, the gurus here may want to know.. Did you tap into the vacuum line for the distrib when you installed the cruise control or the reservoir?

Is it the heat that is affecting me this evening, lack of sleep, or am I actually on the right track for once?
Gurus?
kids1

Correction:..

the line should read "One more thing, the gurus here may want to know.. Did you tap into the vacuum line for the distrib when you installed the cruise control FOR the reservoir?
kids1

Robert,
Is this one of those spherical "cannon-ball" reserviors? I cut one open recently (from a '78 Camaro) and was surprised to find it had an integral check valve inside...
Rob Edwards

I have had some experience with cruize control stuff years ago and problem I had was the vacuum actuator had a rip in it's diaphragm thus loosing vacuum ie sucking air into the manifold. The use of a vacuum reservoir overcomes the rate of leak thus the motor runs better but cc still not reliable. Also explains hard to start.
I have bought many 2nd hand things especially washing machines that have worked like a charm for PO and have just been rebuilt/fixed/serviced. My ownership discounts all this and puts a curse on the device so it fails intermitently at first then ceases to proceed. New things can do this too but are easily replaced with another. I like things made by machines because as soon as humans touch it . . goes without saying.
. . . . . henry
H O

When I connected the vaccume line I tied into the top of the manifold balance tube. I have put a vaccume on the resovoir with my mouth and could not feel anysort of checkvalve operating. This piece came off of a 93 Z28 Camaro, first year for the LT1 engine in the F-body. The car realy isn't that difficult to start, twist the key and vrooom, it takes off but will die if the throttle isn't tapped initialy.This whole thing has puzzeled the heck out of me, it would appear that the engine has reduced its ammount of smoking during hard acceleration also. Perhaps I am to tired to notice anymore, or maby I am finaly loosing it from fixing the gas tank half a dozen times in the last two weeks...
BTW- the cruise control still doesn't work, the unit cost $45 from sears, was still in celephane when I opened it. It had been sitting in our shop attic for many years- it is possible that the heat up there killed the circuit board. For reference- a new cruise from JC Whitney costs $110 for a cheap one.
http://www.hononwah.freeweb-hosting.com/Index.htm

Robert

Most of the newer cruise controls use ICs to control their functioning and can be relatively sophisticated in what they do and what they make of the information they get from vehicle speed and engine speed sensors/hookups. Many older cruise controls used discrete component circuitry (individual components - transistors, capacitors, resistors, etc) and were much less sophisticated in how they operated.

Also, the idea that the heat of the attic might have affected the unit is possible as is the simple factor of age. Most things age regardless of use, tho at a slower rate, but heat still accelerates any aging process, static or dynamic. And, then there is the problem of whether your engine is really at the marginal lower end of the design parameters for this unit. If it was primarily designed for bigger V-8s and sixes, it might not work well with a smaller 4 banger. This is one reason for the use of the vacuum resevior. You are having to provide a steadier vacuum to the unit than it can get from most small 4-cylinder engines.

I have no good idea why you got the symptoms you did with this unit, but it may be just that it is not compatible. But I would also tell you to beware of the newer units as well. I have not had any good luck with the Audiovox units I have tried, not even as good as your luck with this older unit. But perhaps that is just my luck or lack of understanding showing.
Bob Muenchausen

I would like to thank all who responded to my question despite it not making sense.
Robert

Robert. If one can properly frame the question, one seldom needs to ask the question. You, I and many others ask for help because we cannot properly frame the question. All of us have been there in the past and will be there in the future. Thus, when we ask for more defining information, we are beginning to define exactly what the problem is and what may cause such a problem. This is an interactive medium and, thus, we must interact if we are to find a possible solution to any problems posed. Les
Les Bengtson

<<<snip>>>
8)while out driving I stand on the throttle hard and notice that the car is traveling at a higher rate of speed and the engine pitch has changed when the engine runs out of RPMs
<<<snip>>>

Valve bounce? Not good. You might also have an ignition problem which limits the RPMs. The tach is triggered by the ignition system.

9)My tachometer will not read any more than 4500 RPMs
10)I disconnect the resovior and close the vaccume port on the intake manifold.
11)I go for another drive
12)When the accelerator is floored and the engine has run out of RPMs, the car is traveling slower, the
engine sounds like it is turning over more slowly.
<<<snip>>>

There could be an air leak in the system cruise hoses or the vacuum motor. In the first instance, the leak has leaned up the fuel system and it runs better as the the carb settings may be too rich. Also a richer mixture is harder for the ignition to fire.

Your first step should be to give the ignition system a thorough check and tune up including the high and low tension leads, points, cap, rotor, and coil.
Blake

This thread was discussed between 22/04/2003 and 26/04/2003

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