Hard Starting Cold or Warm

DBCooper

New Member
Nov 7, 2006
21
Hi All,

I have a 1991 7.4 Merc 454. its hard starting when the motor is cold(sitting for week) or warm (sitting for an hour or two). However the next day fires on a dime. How can I get this every time?


Thanks
 
I can't say for sure what your problems are but I once had a car that had a slow leak in the float bowl of the carburetor. It behaved just like you describe especailly after it sat for a week or so. The float bowl would empty out so it took a lot of cranking to get it to fire off. No problem after sitting one day because the bowl wouldn't empty out that fast. :smt021

I eventually had to replace the carburetor on my car.
 
DBCooper said:
Hi All,

I have a 1991 7.4 Merc 454. its hard starting when the motor is cold(sitting for week) or warm (sitting for an hour or two). However the next day fires on a dime. How can I get this every time?


Thanks

Welcome to owning a Rochester carb. The reason your engine is so hard to start after sitting for several days is because there is no fuel in the carb. This combined with the mechanical fuel pump means that you keep pumping the throttle lever and cranking the engine until it fires.


It also leaks down after it has been running effectively flooding the engine making it hard to start. That requires you to push the throttle open to get enough air into the carb for it to start and pull it back when it fires.

The fact that it will fire the next day without issue is in line with the fuel still being present in the intake but not to the point that engine is flooded.

(BTW, I hate Rochester and Carter carbs.)

The easiest way around this is replace the carb with another marine grade carb. My preference is Holley. You can get a direct bolt on that will solve these problems and save you fuel. They are around $400 new. If you do a search on this site I wrote about this upgrade a few weeks ago.

-John
 
Funny you should mention Carter Carburetors John, because that carburetor on my car was a Carter Thermoquad that was a combination of aluminum and composites and it never worked right even when the car was new. Even the accelerator pump would fail on those things with regularity giving you a nice stumble off idle. A Holley is a much better carb. :thumbsup:
 
Dave,

I'm with you on that. Holleys are so simple to maintain compared to a Carter Thermoquad or a Rochester Quadrajet. Those companies give carbs a bad name.

-John
 
Thanks all.

I think I can handle the swap of the carbs but I have 2 hesitations.

How do I know what Holley carb to get for my engine and how much configuration and tuning will it need?
 
Thanks John,

I took a look at that thread and it was discussing a 5.4L and I have 7.4L where can I get the Model Number of what I have to look up the comparable Holley?
 
Do you have a spreadbore carb now? Look into the carb, open the linkage and tell me if the two butterfly valves at the front (primaries) are the same size as the back (secondaries). On a spreadbore, the primaries are the size of a quarter and the secondaries are the size of an old silver dollar. On a square bore both are the same. Once you can provide that I can give you a carb model number.

-John
 
So I took a look at my carb yesterday and compared it too the ones displayed in the MerCruiser Repair Manual and it looks like the Holley 4 bbl listed in there, but I could not find any model or manufacturing info on it.

Would a rebuild of my existing Holley carb fix the issues I am seeing?

Thanks again,
 
I got some pictures, Do thses help?
 

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That's not a Holley. It looks like a Rochester Quadrajet Spreadbore. The pictures seem to indicate that the secondaries are bigger than the primaries which is the traditional spreadbore configuration. The alternative is a "square bore" whereby the primaries are the same size as the secondaries.

So, if yours is the spreadbore, the model I would suggest is:

Brand: Holley
Product Line: Holley Model 4175 Marine Carburetors
Part Type: Carburetors
Number of Barrels: 4
Carburetor Flange: Spread bore
Choke: Electric
Fuel: Gasoline
Secondary Type: Vacuum
New or Remanufactured: New
Fuel Inlet: Single
CFM: 650
Carburetor Finish: Dichromate
Ford Kickdown: No
Throttle Linkage Type: Universal
Quantity: Sold individually.

Carburetor, Model 4175, Marine, 650 cfm, Spread Bore, Electric Choke, Single Inlet, Dichromate, Each

Check to make sure this part fits your application


A Quadrajet for your boat.
These Holley Model 4175 marine carburetors include all of the same features as their street cousins--plus a few more that make them adaptable and legal for marine use. Primary differences include fuel bowl vent tubes, sealed throttle shafts, and universal marine calibration. Plus, they're designed to meet flooding and backfire requirements as defined by the U.S. Coast Guard. Other than that, they're typical Model 4175 quality all the way, with a spread bore flange, vacuum secondaries, single fuel inlets, an electric or manual choke (your choice!), 30cc accelerator pumps, and a 5 in. airhorn (unless otherwise noted).

http://store.summitracing.com/partdetail.asp?autofilter=1&part=HLY-0-80552&N=700+115&autoview=sku

-John
 
That doesn't look like a Quadra junk. I have a 7.4 with a spreadboar. The one in the picture doesn't have the top secondary plate and also the secondaries on a Quadra don't have round venturies in the center. The quadra also has a front vent tube that comes up then has a u bent in the end to direct overflow down the carb. My carb also has Quadra Jet stamped right on the throttle linkage side of it.
 
darimont said:
That doesn't look like a Quadra junk. I have a 7.4 with a spreadboar. The one in the picture doesn't have the top secondary plate and also the secondaries on a Quadra don't have round venturies in the center. The quadra also has a front vent tube that comes up then has a u bent in the end to direct overflow down the carb. My carb also has Quadra Jet stamped right on the throttle linkage side of it.

You have a different model on the your 7.4 (big block verse small block). This is either Quadrajet or a Carter. Both were used and look similiar from the top. I agree with your "Quadra junk" persepective. I can't tell from the photos if he has a spreadbore. The only way to know for sure is to get a number off of it or take it off and flip it over. I love that "Quadra junk" comment!

-John
 
I am starting to feel like this :smt021

I don't think mine has an electric choke, this is on a 91 270. would that make since?

Thank you for putting up with me.
 
DBCooper said:
I am starting to feel like this :smt021

I don't think mine has an electric choke, this is on a 91 270. would that make since?

Thank you for putting up with me.

Never a need to apologize. We all own boats. Some of us actually like to work on them which teaches us patience, anger management and cash flow. :smt021

Your current carb has a mechanical choke. There is a small metal rod which attaches to a little metal housing containing a spring on the intake manifold. This spring mechanically opens and closes the choke plate on the carb based on engine heat.

Most marine Holleys use an electric choke which is comprised of "run" wire (ignition on +12v source) that attaches to the choke unit on the side of the carb.

To give you a sense of what you are getting into: On a mechanical scale of difficulty (1 to 10 being most difficult) this job is a 3. If requires some working knowledge, tools, wiring (choke wire), and modification skills for the linkage and fuel line. A qualified mechanic can do the job in less than an hour. If you are not comfortable with the process (especially the fuel part), get a friend who is pretty competent as a mechanic to help, they (we) usually are easily bribed with alcohol. :smt001 It's the best way to learn how to do things.

Please don't take this on unless you are really comfortable with the procedure. Life teaches you a lot of things which is why I pay somebody else to do things I'm not comfortable in doing. For example: climbing trees to do pruning and take down. At 52, with four daughters. an ex-wife and a new fiance; it just seems to make a lot less sense than it used too.

-John
 
PlayDate,

I thought DBCooper had a big block 7.4, anyway I love calling them Quadra Junks. :smt001 I been think of changing over to a holley too. I hate the leak down thing, but have learned to live with it and I know just the right starting touch.
 
darimont said:
PlayDate,

I thought DBCooper had a big block 7.4, anyway I love calling them Quadra Junks. :smt001 I been think of changing over to a holley too. I hate the leak down thing, but have learned to live with it and I know just the right starting touch.

You are right....I have been responding to too many posts!!! For a 7.4, Holley suggests one of the following:

Square Bore: (you need to buy the electric choke kit)

750 CFM
http://store.summitracing.com/partdetail.asp?autofilter=1&part=HLY-0-80537&N=700+115&autoview=sku

800 CFM
http://store.summitracing.com/partdetail.asp?autofilter=1&part=HLY-0-9022&N=700+115&autoview=sku

Holley used to make a 750 marine spreadbore for big blocks but I can't find it listed any place. I would probably use a square bore to spreadbore adapter instead of replacing the manifold especially on a raw water cooled engine. It raises the carb height by 3/4" and evenly distributes the fuel. You also need a throttle linkage plate and fuel line.

Adapter

http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM-G1401&N=700+115&autoview=sku

Linkage Plate

http://store.summitracing.com/partdetail.asp?autofilter=1&part=TRD-2333&N=700+115&autoview=sku

Choke kit

http://store.summitracing.com/partdetail.asp?autofilter=1&part=HLY-45-224&N=700+115&autoview=sku

Once, you get used to the leak down, flooding and hard hot starting you can get along with Q-jet.

I will tell you that on my boat (twin 7.4s) with a Navman Fuel computer, I went from .7 miles per gallon (26 mph) to .76 miles per gallon at (26 mph) with this change (adapter plates). In real terms, I pay $3.50 per gallon for fuel which means I was burning fuel at $4.55 per mile. The new fuel savings is close to $.20 per mile or $200 per 1000 miles.

Over time the upgrade(s) have paid for themselves.

Sorry for the confusion,

-John
 

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